Author's Note: A section dedicated to everyone's favourite life exterminating abominations, expect to see more updates as the war rages on.
History
Let's get this out of the way: We have no bloody idea. The Reapers are ancient, ageless abominations and many of them have been active since the dawn of the universe for all we know. What we do know comes from first hand accounts of the Lich's who saw the end of the last cycle, and most of them have been lost in a timeless abyss of madness. The Reaper's earliest believed feat was the construction of the Citadel and the Mass Relays which act as a combination slingshot/catcher's mitt for space ships enabling FTL travel without relying on the chunks of warped reality we normal people rely on. This is part of a trap established by Reapers, through the Relays the Reapers can control and limit the flow of ships that rely on them and are instantly superior in terms of technology compared to the races that rely on bits of technology that people have been able to gleam from studying them. Lil before the extinction of the Dinosaurs the buggers swooped in, killed everything with anything more interesting than a photocopier. The process involved is a little more complicated than that but is better covered elsewhere. What we do know is that they were interrupted in the process by the simultaneous detonation of 90% of the Mass Relays which has resulted in the Lich boast of "Killing more Reapers on that day then what have ever died before or will ever since." I don't know details of the exact body count but I'd rather not resort to the destruction of all life in the galaxy to pull it off again. Moving on. We don't know what the Reapers have been up to in the 65 Million year gap precisely, we are fairly confident that with the exception of a monitor in the form of Sovereign and the Collectors the Reapers have largely made themselves scarce in the galaxy. Now keep in mind that all of the above, including the ridiculously huge gap between the last cycle and now that we have no idea what they were doing, is less than a hundredth of the potential history behind the Reapers.
Biology
Which I know is odd to say about a race of living space ships, but here we are. Reapers are synthorganic organisms. According to the Lich's they cherry pick through the various races searching for "worthy specimens" compatible with their technology, melts them down into goo, mix that together with a nanite cocktail and a Eezo mixture and then pumps that into a type of artificial womb which gradually grows into a newborn Reaper using millions of people as the embryonic goo. If you buy into Reaper propaganda, strongly discouraged by the higher ups FYI, then this process preserves the mind, intelligence and memories of the individuals in question as part of the nascent consciousness of the Reaper. After a Reaper has been fully grown the Cephalopod like shell we're all familiar with thanks to Sovereign is built around it and tada! You got a brand new eldritch destroyer of civilisation. Also thanks to Sovereign, we got an idea of how they're built and the best way to take one apart.
A Reaper Shell is composed of a Neutronium Super Conductive shell. Neutronium refers to material that has been shrunk, for lack of better term, to it's smallest possible form. The space between Protons and Electrons in a Atom often uses terms like Baseballs and Stadium's to describe the space between the two, Neutroium is what happens when you take the two baseballs and put them on the pitchers mat. The Union has had access to material like this for centuries, ever since the Lithoids started pulling it out of black holes, and even for a while used it to make ships until Dragon Scale Armour became more cost effective. The difference is that instead of minerals the main material used in constructing Reaper hulls is a mixture of super conductor alloys. By themselves this super conductor alloy is only a fraction of the strength compared to other Neutronium materials, but is bolstered by a powerful Mass Effect field. Experiments from the Scientific fleet trying to dismantle Mass Relays has confirmed that active relays are seemingly 'locked' into position on a molecular level. Every individual molecule of the Relay is pinned in the exact location, by what we've been able to confirm is a Mass Effect field, any amount of force that is capable of scratching the surface of a 'molecularly locked' object would likely shatter a planet if it was aimed at it. The good news is that it is possible to destroy this material, The Union believes the most effective strategy is to first drain the Reapers of power, then use a molecular disentegrator to tear the Reaper apart molecule by molecule. Simple in principle, nigh impossible to execute unless you have a fleet of ships given the sheer scale of the Reaper's power generating capabilities.
In terms of fire power, Reapers prefer what the Council has termed the Thanix Cannon as it's main gun, a mass of liquefied ferro-fluids consisting of a mixture of Uranium, Tungsten and Iron kept suspended in a Mass Effect field and fired with ludicrous force at roughly a tenth the speed of light after having it's weight dialled up as high as possible. The Council are attempting to reverse engineer smaller versions of these to strap to their own ships, to questionable effectiveness. Aside from that the secondary 'Leg' mounted weapons are the Black Hole cannons. A little on the nose for a name, basically these things fire miniature unstable artificial micro singularities at middle distance away from the Reapers which strip the armour off of enemy ships, softening them up if the Thanix hasn't actually manage to do them in yet. The Reapers apparently don't fear firing the damned things at close range either, probably because their hulls can't be squeezed down any denser than what they already are.
Most Startling off all though, they have innate psionic abilities, which aside from the expected applications of telepathy, mind control and telekinesis, a Reaper may 'assume direct control' over a husk significantly amplifying their combat prowess in the process as well as allowing the monsters to take command on the battlefield.
Any particular differences between Reaper "Variants" is better covered in their Military. Speaking of...
Military
Reapers come in two flavours, as far as we know.
* Destroyer Class: A 'Small' Reaper may be deployed planet side, aside from casually trampling over buildings, military vehicles and civilians their main job is acting as a broadcasting station for the Reaper's ultrasonic Indoctrination signal converting the masses to get on board with the idea of jumping into Reaper melting pots, or worst yet transforming evacuating civilians into potential suicide bombers and saboteurs in anticipation of later attacks on other colonies.
* Sovereign Class: Referred to as such until further, notice, these are the catch-all term for the Reaper vessels in the same weight-class as Sovereign, and hence most of the above description applies to them, seems to be built for space combat specifically, and apparently wrestling Dreadnauts.
* Harbinger Class: Unknown, possibly mythical. Stories from the tail end of the last Cycle describes a colossus class ship responsible for ending entire civilisations single handily. If it does exist, I hope I'm dead before it shows up, the old texts describe planets of husks left behind in their wake, which leads to the next topic.
Reapers don't waste resources, and this includes the dead. The same nanite cocktail they use in making their synthorganic cores are just as readily available to use in mass production of ground forces, once injected the individuals in question are warped and mutated, weaves of synthesized material appears inside of their bodies enhancing physical strength and biotic potential if available, each and every one of them a ready made horror unleashed upon the galaxy. We've seen this done in two ways, one is through the Dragon's teeth, mechanical devices which skewers a corpse or a still living person and injects them with this nanite cocktail, the Reapers apparently leave them buried like mines so that they can skewer entire armies ala Vlad the Impaler style. The second way is the seeker swarm, insects loaded to the brim with the nanite cocktail who seem to exist for no other purpose than to bite people and spread their infection. Sadly, between the Mercenaries he hired and the civilians that were onboard the Citadel when the conduit opened up these bastards come in every flavour available in citadel space, and as the war with the reapers rage on we expect the number of variants to increase over time. Below we see the fruits of the Tebrid Scientific fleet's efforts to catalogue and classify them. Apologies to the families of the victims, some of the names the slugs have come up with can be considered... offensive.
* Abominations: Volus Husks, the organic matter inside of the Volus's bodies have been converted into a highly explosive material kept inside of a pressurised shell, during the battle for the citadel other husk variants would sacrifice themselves soaking up slug fire so that these things could reach hard targets and light themselves up tearing entire buildings down in the process and turning entire groups of soldiers into shrapnel. If you see these merry suicide bombers in the future shoot them while they're still far away, preferably while they're still in the middle of the enemy forces.
* Banshee: Asari Husks, elements of the Asari brain chemistry have been rewired while cybernetic tissues and have spread through the body, In comparison to a normal Asari Banshee's see a 70% increase in height and their stomach's have been bloated into a churning biotic furnace. Overall biotic ability sees an approximately 200% increase in effectiveness which aside from dramatically augmented barriers and the usual slew of bioitic abilities also sees them using biotics to enhance the force behind their blows in melee, which usually manifests as a set of stabbing fingernail claws. What's even worst though? The Banshee gets it's name from a shrieking ultra-sound laced voice that carries undertones of a Reaper indoctrination signal.
* Brute: Krogan Husks, the usual size and strength of the Krogan have been put on steroids with forced muscle growth. The Hump on a Krogan's back contains a dense emergency supply of Nutrients, the Husk process took advantage of that, use the nutrients as fuel to force extra muscle growth on top of cybernetic muscle tissue forcing their way through. The Brute is trapped in a perpetual state of Krogan Blood Rage that sees them charge through enemy lines like a walking battering ram, and blocking shots from gunfire like a giant all natural siege engine. If they don't trample you to jelly, they'll blow you apart with grenade launchers cause yes, the Reapers give their monsters guns.
* Cannibal: Batarian Husks, the Batarian's blatant worship of the enemy has not made them any less inclined to convert their followers into more of the abominations, and on the contrary it's quite probably that they've been stocking up on them for a long time. As the name implies, Cannibals wander the battlefield looking for the corpses of their fellow Husks and... absorbing them into their own bodies. We aren't sure how this process works, we believe the Cannibal re-energises the corpse's cybernetic reaper implants, linking them to their own and absorbing or absorbing into any remaining intact components, repurposing organs into sacks of synthorganic generators and limbs into rifles, we call these Creatures Scion's. As a rule of thumb 'shoot on sight' is the recommended course of action with Husks, but if you see a Cannibal, shoot it while it's small and weak, if you see one in the middle of integrating with a corpse, shoot it immediately until both it and the corpse stops moving.
* Collector: Prothean Husks, the Prothean's themselves have long since gone extinct, but these things are still alive and kicking, chunks of Prothean DNA have been removed, alongside several internal organs, replacing them with various cybernetic implants. We presume that they were stripped of 'Unnessary components' for the sake of preserving their DNA for future generations. Collector's are naturally equipped with fully functional insect wings, are fully psionically and Biotically capable and are equipped with synthorganic arms and armour, in battle they tend to make up the majority of the actual ground troops.
* Ghost: Salarian Husks, the Salarian's remain intellectuals, even as undead synthorganic abominations. Built in Mass Effect fields manipulates and pushes light away from their bodies creating a invisible assassin type creature, fingers once used to tap data-pads and build scientific marvels have been turned into six inch long vibrating blades that stab and spread instead their targets. Lighting quick reflexes lets them read the flow of the battlefield and make quick paced decisions to avoid allied or enemy gunfire and strike at the most successful soldiers on the enemy lines.
* Harvester: Hanar Husks, Hanar who have been converted to float along the battlefield on synthorganic biotic generators while their innate psionic potential has been drawn out, amplified and dedicated to the use of moving along their now significantly lighter bodies. This by themselves do not make them a significant threat, it's only after you then add the multiple grasping limbs of a Hanar that has been further augmented with hooked graspers capable of latching onto other husks and sharing their weight reducing biotic field with them that the true horror of a Harvester comes to pass, Brutes flying over the battlefield and dropping onto your position directly, multiple Marauders shooting at you from the air, Abominations casually dropped on your embankment like they were grenades. And when they're done, they're grabbing you next, dragging you kicking and screaming out of cover and into the fire line, or worse.
* Hunter: Vorcha Husks, the Hunters were found roaming about the citadel in a pack, swarming defenders with sheer numbers, we're not sure how they did it but the science fleet think the Reapers took advantage of the low genetic diversity of the Vorcha, their hyper adaptive biology and some kind of telepathic link to create a type of... group adaptation? One or two drops from slug fire and the rest grow bullet proof plating, some more gets set on fire and the rest learn how to use a biotic field to smother the flame, they get electrocuted, some drop the others learned how to insulate themselves, they get frozen and then the biotic field starts to vibrate to thaw them back out. Don't let a single one of these things get away, every trick that works on them this time might not work on them a second time.
* Marauder: Turian Husks, the Legs of the Turian's have been augmented by several strands of synthorganic muscles given them an impressive jumping capability while their eyes have had several layers of optics added on top and one of their arms have been turned into a Synthorganic rifle while the other hand has been turned into a barbed grasping talon. These extra features in combination makes the Marauder a highly effective sniper, capable of aggressive guerrilla tactics and a tactical over view of the battlefield helping to coordinate their fellow crimes against sapient life into a organised military force, if the Salarian's remain intellectuals as Ghosts and infiltrators then the Turian's remain disciplined soldiers and expert commanders.
* Praetorian: Unknown Husk, A massive four eyed six armed abomination closely resembling the Reapers themselves that floats around the battlefield like a giant weapons platform, DNA tests confirms the heavy presence of not only Prothean but also Batarian DNA, like the Collector's just had the extra parts lying around and decided to make use of them for something.
* Ravager: Rachni Husks, if the Brute is the shock and awe monster designed to tank hits and crack open walls with brute strength the Ravager is the Mid range grenade lobbing heavy weapons platform designed to fill said newly punched hole with explosives. The Rachni's four legs makes for a surprisingly effective base for the heavy artillery that has been strapped to their back, a combination of acid and unstable chemicals makes for explosive mixture after exposure to air releasing deadly gasses often into a confined space. However the Brute is not the only deadly two hit punch these things are a part of.
* Swarm: Rachni Worker Husks, Rachni largely come in two variants, the Ravagers are born from the workers, the Swarm is based on the smaller Rachni workers, the ones responsible to dissolving and reshaping structures in order to build the Rachni's hives. When the Ravager dies it releases a mass of these merry little acid sacks to tear through whatever bloke was responsible for downing the Ravager.
* Tyrant: Elcor Husks, in the Elcor the Reapers found an ideal mixture of brains and brawn. Elcor are physically massive, like sticking two Krogan next to each other, a consequence of a home world with a unusually massive gravitational pull, another by product of this is a extremely careful and considerate nature, you would be too if one wrong step means instant death. The Reaper's took advantage of this, in one arm is a mass driver, strapped to the other one a full body shield, in the middle a physically massive beast to rival the brute but with a head for tactics on par with the Marauders. Oh and the voice synthesizers are still fully functional, so now you giant physically massive husks, armed with a shield, a anti material rifle, and on top of that it can shout insults and propaganda at you too.
in short, the Reapers ground troops are as diversified and varied as the number of enemies that they face. Their technology is as advanced and powerful as one would expect from a collective that has been developing it for as long as they have been. Their numbers... their numbers are still probably large enough to cover the entire galaxy, we know what tactics work on them but said tactics are so hideously expensive that we'll run out of ships before the Reapers do. The only consolation is that the Mass Relays simply do not exist in Union territory so the Reapers are either going to have to adapt to our way of doing things or rely on the hideously power hungry worm hole generators, which they might just be able to do anyway for all that we know. Reality has been broken in Union space, the lane-ways and the worm holes are not only open but near as we can tell here to stay, if the Reapers mean to continue the tried and true tactics they've developed in the future then they're going to have to make some improvements.