The relay network:

Mass relays form a vast network across the galaxy. Only a tiny amount of it has been explored though. Given the spatial distribution of known relays, it is suspected that as much as 1% of all the stars in the milkyway may have a relay orbiting them. That would make over 1,000,000,000 relays, with the number of stars in the milkyway ranging between 100,000,000,000 and 200,000,000,000.*

This leads to some other questions: How many other spacefaring civilisations might be there? Are there other hub stations, other Citadels?

There also seem to be some gaps in the relay network. To get from Palaven to the Citadel for example, one has to take a detour, because on the direct route a relay seems to be broken or missing. This is the reason why it took the Turians so long to make contact with the rest of the galaxy. If this is due to natural causes or if these gaps have been made artificially is unknown.

Mass relays create a "tunnel" through spacetime between them. Objects within the tunnel have no effective mass and traverse it instantly. The technology behind this is still not truly understood.

Spacefare and Starships:

Like nearly all vehicles, starships have an element zero core which enables them to alter their mass. In spaceships the core is big enough to nullify their mass completely, allowing them to achieve speeds faster than light despite e=mc².** Ground to orbit transportation is done by cargo shuttles with eezo cores, allowing for cheap and decentralized delivery of goods to ships and space stations. Ships can land on planets if needed, but only frigates and other small ships usually do so. Ships are powered by fusion generators and use fusion hydrogen torches. Military ships amd some others whose owners can afford it also have antiproton drives. The needed antimatter is difficult to make though, so supply is limited.

Warships of cruiser size and upwards are armed with a spinal mass accelerator as their main weapon and some secondary mass accelerator turrets. Some frigate types also have a spinal gun, but mostly they sport disruptor torpedoes or other missile types, which can be equipped with fusion warheads or varying sorts of more conventional armaments. GARDIAN (General ARea Defense Integration Anti-spacecraft Network) laser systems serve as missile and fighter defense, and frigates have been known to use their GARDIAN systems for ground attack runs on several occasions. Since Lasers move at lightspeed, they cannot be evaded by anything moving slower than light. Diffraction limits their range, and continous use will overheat them.

Kinetic barriers are the first line of passive protection, followed by different forms of composite armor. Other than barriers though, any known armor will be breached by a shot from a spinal gun.

Disruptor torpedoes are the second most dangerous weapon in space combat after spinal guns. They temporary create a small singularity on detonation. Thus armor does not provide any defense, since the singularity warps spacetime itself in a volume around it. Barriers are the only viable protection, but a well coordinated torpedo strike can overwhelm any known barrier. They are very vulnerable to GARDIAN fire though and so are only used at close distances or en masse. Since they need eezo to work they are also expensive and almost exclusively used by regular military forces who can afford them.

Frigates are fast and agile, though lightly armed. They are utilized for patrols and reconnaissance. In larger engagements they form wolf packs to harass enemy ships or to screen the fleet from fighters.

Cruisers are workhorse of military fleets. They lead patrol groups, screen dreadnoughts in larger engagements and fight their counterparts.

Dreadnoughts range from 800 meters to one kilometer long, with a main gun going through about 90% the length of the ship on average. An 800-meter mass accelerator is capable of accelerating one twenty-kilogram slug to a velocity of 4025 km/s, which is 1.3% the speed of light every two seconds. Each slug has the kinetic energy of about 38 kilotons of TNT. The firepower scales with the length of the gun, so bigger dreadnoughts have stronger guns. Due to the high rate of fire heat builds up fast. Since heat management is tatically important, heat venting and -storing technology is continously improved. At present, a Dreadnought can fire dozens of slugs in quick succession before it has to stop firing for while, which usually means it jumps to FTL and vents heat in a safe location.

The Treaty of Farixen limits the number of dreadnoughts any of the signing powers can possess to a proportion of those the other powers have. This is meant to maintain balance and to prevent an arms race that could destabilise the political climate and ruin economies. As of 289, or 2525 by the UEG calendar, the turians have 37 dreadnoughts, the asari have 21, the salarians have 16, and the batarians have 6 with another under construction. In 2525, the UNSC has 24 Marathon-class cruisers which are considered the tactical equivalent of dreadnoughts.

Ships with element zero cores are highly maeuverable and due to kinetic barriers capital ships can survive hits from mass accelerators. Space battles between such ships are a highly dynamic affair with rapidly shifting formations. When the kinetic barriers of a fleet are weakening, the high firing rate of spinal guns means that ships can be destroyed in quick succession. Thus the losing fleet usually retreats into FTL as fast as possible to avoid wholesale destruction.

Ground vehicles and infantry:

Combat vehicles are usually equipped with mass accelerator cannons and missiles. They are protected by composite arrmor and kinetic barriers. Their element zero cores make them highly maneuverable, and most combat vehicles, even main battle tanks, can be airdropped by ships or shuttles.

Infantry uses full body armor of varying protection value, broadly divided into light, medium, and heavy armor variants. Guns are handheld mass accelerator that fire small slugs at very high velocities, giving them a surprising amount of firepower. Ammunition is cut off an ammoblock that is big enough for thousands of rounds, making reloading not a concern during battle. Weapons need to cool down from time to time, but short, controlled bursts and weapons of good quality allow for nearly continous fire. Different modifications can be made to the ammunition with an addon to the weapon, ranging from emp rounds to the infamous polonium ammunition for a wide varety of situations. Mods can be easily changed even during combat, adding valuable versatilitiy to standard infantry gear. Heavy weapons include grenade and missile launchers that come with a wide range of ammo types.

Notable race specific information:

Turians: The Turians have the largest fleet and the largest ground forces in the known galaxy. Their ships have slightly more powerful weapons than those of other races and their troops are known for their discipline even under the most difficult circumstances.

Asari: Some say they are the technologally most advanced race, closely followed by the Salarians. Their kinetic barriers are the strongest known and they introduced cyclonic barrier technology to the wider galaxy. While they have fewer dreadnoughts than the Turians, they field a big fleet of other ships. All Asari are biotic and their commando soldiers can use this to devastating effect. Their long lifespan means that they have some of the most dangerous combatants in the galaxy, rivaling the famous krogan warmasters.

Salarians: Salarians favour espionage, sabotage and ambushes over open warfare. Their fleet is still relatively small compared to those of the turians, asari or humans but their ships are equipped with expensive cutting-edge technology to great extent. They also have more stealth ships than the other major powers. Their intelligence service, the STG, is universally respected and feared. The STG is part of the salarian military and their agents have often decided battles before they began.

Volus: The volus are an economic powerhouse with a small military. They are a military client of the Turian Hierarchy and their forces are integrated the Hierachy command structure. The volus are methane breathers, so they have to wear pressure suits in most environments. This makes them vulnerable in infantry engagements. Due to their diminutive size they also lack in physical prowess. Their military excels in one field though: Volus bombers are the most formidable ships of their class and usually support turian ground operations or supplement attacking spacefighters with their durability and firepower.

Batarians: The isolationist Batarian Hegemony does not disclose much about its military, though they abide by the Treaty of Farixen. There are persistent rumors that they have a huge arsenal of devastating chemical and biological weapons. The Hegemony does not comment on these rumours.

Elcor: The elcor have a rather small fleet that primarily protects their colonies and trade routes. Their merchant fleet is much bigger though, and their trade with the other races has led to good relations with them. Elcor are slow, but very strong and resilient. While they dislike infantry engagements in the open, elcor marines equipped with thick armor and VI controlled heavy weapons make formidable opponents in urban and shipboard combat.

Hanar: Their fleet is comparable to that of the elcor, with a strong emphasis on missile swarm tactics. Their many tentacles make them able to handle complex controls with higher speed and precision than other organic lifeforms and hanar pilots are famous for their skill. Physically unimposing and fragile, they make heavy use of combat drones in ground engagements. Their infantry weapons fire exotic ammunition types the hanar have developed to make up for their lack in raw power and often surprise their opponents. Many drell serve in their military and drell assassins are usually supplied with hanar ammunition, adding to their fearsome reputation.

Krogan: The Krogan have no fleet and no central command structure. They are divided into clans which maintain their own respective forces and many krogans work as mercenaries throughout known space. Krogan are strong, resilient and most are seasoned veterans, having grown up in a warrior culture their harsh homeworld of Tuchanka. Individually, krogan warriors are the most effective infantry known. Krogan battlemasters combine the combat prowess of their species with centuries of experience and training, making them enormously dangerous individuals.

*This is mainly to clarify how huge our galaxy actually is, since people often seem to be ignorant of that fact. "They only control 1% of the galaxy, that's so small, yadda yadda..." It is actually a worldbulding mistake because it would make for a truly enormous civilisation, and what we see in the games points to a much smaller one, more like a few thousand inhabitated worlds overall, including rather small colonies. For comparison: The IoM in 40k has "only" a few million worlds, and it's huge!

**Of course eezo violates thermodynamics and a bunch of other things, but one has to make concessions, as with artificial gravity and slipspace.

Author's notes:

Zernio: Most of this is not going to happen. As stated, this is before the human-covenant war, and the Covenant does not exist in this alternate universe. First contact happens in 2525 and the UNSC will have the tech they had at that point in canon, aside from the improvements I made and described. Ideas and suggestions are welcome though.

F13D:

1. The 600 ton slugs are way, way too slow to be used in combat against spaceships. This is a worldbuilding mistake in Halo canon, where ships move faster than the projectiles they fire. Which is frankly absurd. I changed the numbers to smaller, but faster projectiles that have the same firepower, as you might have noticed, but a much higher and more realistic range. And no, that 30 km/s is not a typo as some people claim. I ran the numbers on a 600 ton slug moving at 30km/s to see how much kinetic energy it would have. The numbers check out at the equivalent of 64,53 tonnes of TNT. So whoever came up with the original stats of the MAC did the math as well. Definitely no typo. My new numbers are also correct by the way, as are the canon mass effect numbers. I checked those as well to be sure.

A went out of the window for similar reasons. It is said to be a titanium-steel alloy, and later titanium-tungsten. Real life tank armor is made of composites far superior to that crap since the 70s. I see no reason why they would be using titanium alloys for armor over 500 years in the future when we have better technology today. And an interstellear civilisation should be able to produce composite armor materials in sufficient amounts to equip starships.

3. "So this is not really a crossover". That's not yours to define, though you are entitled to your opinion. But so am I, and I happen to be the author, lol.