Author's Note: Updated the Batarian page. Now that I've revealed their whole Religion thing I figured it's only appropriate
Overview
The Batarians have the misfortune of being the first citadel race to encounter the Stellar union, and similarly the Stellar Union has the misfortune of encountering the Batarians. So now that the first impressions are out of the way let's all skip to the part where we go into actual information. Make a write up about all the relevant information and send it back with their former masters. Admiral A. Shepard
Biology
Batarian's are a humanoid race possessing average physical strength compared to a unaugmented human with their most notable physical trait being four eyes able to see in multiple spectrum's of light, their complexions have ranged from light teal, to greenish yellow to redish brown and they speak with deep gluttoral voices. They typically favour extremely hot desert-like worlds relatively close to their system's sun.
Culture
Batarian society is built on a strict caste base system, at it's lowest we have the slave caste consisting mostly of disposable work force, and if they're lucky, sex slaves. Such individuals are outfitted with surgically implanted slave controlling devices which include a button to induce great agony into their nervous system and a explosive device designed to kill them if they are no longer worth the effort. This is also the caste where you'll see non-Batarians living on Batarian worlds which includes Asari, Elcor, Salarians, Turians and Vorcha. While in theory it's possible for a Batarian to work their way up through the caste system there is no known record of any non-Batarians achieving a rank higher than Slave. There is however an unofficial list of races not to be taken as slaves: Volus, too physically small and weak to make a labourer and the need for their pressurised suits make them too high maintenance to take as slaves. Quarian's for much of the same reason as Volus, certain less scrupulous Quarians travel into Batarian space on their pilgrimages in hopes of obtaining bits of technology that the Batarian's obtain in their raids, the Batarian's for the most part tend to be welcoming to the Quarian's and tend to treat them on the same level as their Labour caste. Krogan, no one in their right mind is insane enough to try and attach a pain button to a Krogan, they make better soldiers than slaves anyway.
Despite what many believe, the Soldier caste does not receive a lot of recognition in Batarian society, something about a preference to not arm their citizens for an uprising, which includes strict laws about not taking slaves for themselves, more on this subject below.
The labour caste which contrary to what the Batarian's themselves claim are the ones who get the majority of the economy building work done in the Hedgemony on account of the fact that they are the ones who receive the actual training to get things like ships assembled and buildings thrown up.
The Merchant caste is when we start to see the Batarian's treat their own people with respect, even though that is said they are more often than not simple pirates responsible for scooping up colonies and dragging their people back to the slave worlds to die.
The Governor Caste, not unlike most politicians, is both one of the highest ranked and the least respected out of all the castes by their own people, officially the heads of the Colonies their main job is supposed to be maintaining relationships with other people. In reality considering the way that Batarians treat literally everyone, their own people included, to call these dart board targets anything more than figure heads is a bad joke.
The Priest Caste is probably the highest ranked caste in the entire Hedgemony, responsible for interpreting and enforcing their religious texts the Pillars of Strength. They have the most influence of all the other castes and any would be leader of the Hedgemon who tries to do their job without their support can expect to receive a harpoon to the head while he's in his office. Prior to the first contact war most non-batarians aren't really privy to that much information about what the Pillars of strength are or where they actually come from. So it turns out that what the Batarian Codex doesn't talk about is how their religion is built around basically worshipping a Dead Reaper that when loosely translated from Kharsum means 'Leviathan of Majesty.' As if being a bunch of colossal assholes wasn't already encoded into their very DNA by thousands of years as slavers the current theory is that life on the planet first emerged from the leaking organic matter that was coming out of this thing and theoretically further helped along by it's influence. The 'Pillars of Strength' describe the Citadel as the palace of their gods and that it's the Batarian's sacred duty to seize it at all costs, as well as a slightly less sacred but still vastly important duty to send a steady supply of slaves of all the races that they can directly to the reapers, presumably as part of their experiments to figure out which races they should target whenever they eventually get here.
The Hedgemon is the single highest ranked authority in the Batarian Hedgemony, responsible for controlling the colonies, laying down the law and is the sole authority with total control over the Batarian Navy and whom can declare war on the rest of the galaxy.
If you want to talk about actual religion the Batarian's place the most emphasis on their eyes. There is an afterlife for them but it's believed that they will be denied it if their eyes are damaged in any way prior to death necessitating complex funerary rituals. Aside from that? Honestly just Divine Enforcer the whole thing, speaking as the person who reads these things so that you don't have to there is nothing here worth keeping.
History
An odd point of pride for the Batarians is the source of their lower standard of technology in comparison to the other races. Like most of the other Citadel Races they achieve spaceflight by excavating the ruins of the extinct post FTL race the Protheans who once lived on their home world. What made the Batarian ruins unique is that their ruins were partially destroyed in an Earthquake which gave them less to work with than the other races but still made it just as far. Less talked about is the current source of their second rate technology, bought second hand on the market or stolen from the various colonies that they've raided, their heavy industrialisation be damned they build very little of their own ships beyond tearing apart and putting back together the works of others. The Batarians was one of several races who simultaneously made first contact with the Citadel nearly 2400 years ago. However it is believed that before joining the rush of individuals attempting to establish peaceful relations with the citadel they first raided several of the other races colonies for slaves. The truth of where these colonies went, and many colonies since, is one of the worst kept secrets in Citadel space but the Batarian's have fought tooth and nail to keep their right to enslave others sighting cultural rights and other legal pedantics to keep the council in line. There's also the fact that at any given time they have several million hostages that they can kill with the push of a button to use as meat shields and the longer the Council let's this go unanswered for the worst that they look. Thus this stalemate has continued for almost 2400 years... and then the Hyperlane opened up.
Currently we shouldn't expect these guys to make a comeback soon. Upon arrival in the Terminus system as it's known one Admiral Shepard became so thoroughly disgusted with the entire government she declared the Batarian Hedgemony as a whole a hostile force and authorised that they be dealt with extreme prejudice. A Lich Necromancer was dropped onto the nearest world along with the Dragoon contingent and told to manage the mess that was the planet. Undead former slaves subsequently rose in mass and and revolted. Reports of course made it off world, the frequent jamming be damned, but no one took it seriously, even after the first planet was taken everyone just assumed the footage was just a terror tactic to scare the populace into submission. Then the suit skeletons dropped their graveyards out above the worlds that they had yet to invade and started dropping their real troops down to take them out. Currently the former Batarian territory is acting as a neutral zone between the council and the Union, casualties are unknown at this time but thus far? All of the former slaves have been taken off world, Batarian's included. The Military is a sad and pathetic shadow of what it once was. The Merchants are now out of jobs and most of them are in prison. The Priests have been stripped of their former religion. And the Hedgemon has the unfortunate task of trying to pick up all the pieces and put them back together again.
Military
Batarian technology is typically a step below that of the rest of the citadel races. The worst kept secret in Citadel space means that no one but the most reviled are willing to trade with them, or Salarian Saboteurs. Ships are either assembled together with extremely low standards, or stolen from the colonies that they raid and then repair. Occasionally we see both, sadly attempting to make ships out of a bunch of spare parts that were never really meant to go together means that these things are worth F all in a real fight. On the other hand Batarian Soldiers, particularly their SIU special forces, are held to a much higher standard. Many Batarian worlds are full of harsh terrain, jungles, deserts, the point we're leaning to here is that it's hot. During training SIU candidates are given largely outdated equipment to use to conduct their training, they're heavy, they're inaccurate, they only fire one shot at a time, and they can punch a hole through an inch of ceramic steel, facts that made it so that they had to live off of the land hunting the big cat like creatures native to their jungles. Then when the time comes to deploy them in actual combat they're given the latest weapons the Batarian's can get their hands on and the dramatic augmentation of technology dials up their performance. It has been reported that for each Batarian sniper that fell defending the Batarian worlds a hundred Immortals were killed, usually shot through the eyes a particular point of contention for the Lich's. Of course the Lich's were all too happy to confirm the rumours of their necromantic abilities after this and needless to say the defenders were quickly overwhelmed. I would like to say that their attempts to buy extra helped served them well but the mercenaries were finished off faster then the soldiers were. At this point there's no point in talking about the Batarian's Ramshackle navy, the aforementioned predisposition towards looting means that it had very little of a unified standard, and now it's been effectively wiped out.
Conclusion
Honestly just put up a quarantine bubble around their world at this point. Let them stew in their juices for a few hundred years. Intel Analyst P. Harris