Author's Note: Actually forgot about this one.
The unfortunate and rather unflattering truth of our situation is that we have some severe damage control that we need to conduct with regards to the Unbidden doctors. I want everything that you and our intel experts have on them. Admiral S. Hackett
Biology
Biology might be a technically inaccurate term. The Unbidden are a race, and I use the term loosely, of energy beings that have only relatively recently pushed the boundary into solid matter. Their bodies possess neither bones to support them, muscles to move with, or any internal organs that we can find to enable bodily functions. Let alone an organic race, I still do not even know if these things are alive, and certainly not in the same way as a normal living being is. Unbidden reproduction requires that they siphon a large amount of psionic energy from creatures that are alive, at least on par with the intelligence of small animals, and then condense that gathered energy together until they have something resembling solid matter.
Both the Unbidden themselves and all their technology, possibly even everything native to their dimension, is composed entirely of a glass like substance known to us as Zro, named by the Lich in Reference to Zroni their progenitor race. Zro is extremely delicate but extremely useful, it's pivotal in the construction of grey boxes, psychic amplifiers/dampeners, energy shields, holographic projectors, improves the function of Jump Drives and when ground up can be used as a recreational psychic enhancing drug, the Unbidden also use it as the main material in the construction of their ships. The fact that Zro has so many uses only lends further proof to the idea that it is the primary component in their dimension.
The Unbidden have several 'Variants' that have cropped up in the galaxy after they were unleashed on the Reapers, perhaps after spending so long in this dimension they are learning to adapt themselves by integrating with materials that they've come across. There are five known variants of Unbidden, each of them named for caster Classes in Fantasy role playing games, partly as a reference to their otherworldly nature, but mostly as a flight of fancy.
*Warlocks: "Pure" Unbidden, Warlocks are the baseline variant with none of the fun extras that the others have. Sucking the life out of the enemy seems to be their default means of attack with said energy being used to fuel the manifestation of disentegrators or familiars, occasionally shields when they can think of it. In large numbers these things have even been known to create whole space ships indicating their capacity for building complex masses.
*Wizards: Unbidden mixed with Eezo. The vast energy manipulating power of the Unbidden is combined with mass effect inducing Eezo to create a deadly combination of complex Mass Altering fields capable of Lifting, Slamming, Pulling, Pushing, Warping or tossing out a Singularity. They're harder than most other Unbidden too, the effects of the constant mass increasing field that they're under has resulted in denser Zro making up their bodies resulting in the tanks of the Unbidden forces.
*Druids: Unbidden mixed with Thorian Spores. Two hyper adaptive lifeforms stuck together as one. The Druids tap in the Thorian's psychic network, giving them the power to use a rather nasty mental attack, though the Thorian's passive regeneration tends to handle it. The real threat here is when the Druid is able to override the will of animals of the Gestalt and use them as a means of attack, swelling the potential number of enemies massively.
*Clerics: Unbidden mixed with Exotic Gas/Plasma. Not quite sure how this one works, it's possible that they had to learn pyrokinetics to regulate the red hot flame of plasma that their bodies are constantly giving out. Regardless of the composition of their bodies the Cleric's command fire, ice and electricity with disturbing proficiency as those who have been fighting against them, and trails of ashes and shattered frozen soldiers, can testify.
*Bard: Unbidden mixed with Omni-gel and fabricators. We believe this is an experimental Unbidden variant that was created by Henry Lawson, we're not sure about how he did it or why. Working theory is that an Unbidden was forced to feed it's zro into an Omni-gel fabricator as it was trying to form another of its kind. The construction, for lack of a better word, for this Unbidden as a result is radically different from any other Unbidden that we have on record. If normal Unbidden are masses of unrefined Zro then this one is several interlinked computer systems that have been assembled into a semi-humanoid form. It's also important that we keep this Bard alive since it seems to be only one that has some form of rudimentary communication with us.
Culture/History
Very little of which we can decipher. Unbidden thought processes are seemingly almost completely alien to this dimension, at most they can comprehend passion, or rather strong emotions, merely yelling at them is not enough you have to sink emotion and passion into what you are saying before they'll understand you. The one Unbidden we've been able to talk to, the Bard known to us as the Wanderer, communicates entirely music clips for instance, as if the files in question contain the passion from the artists. What we do know is that we've screwed up. Unbidden first became aware of this plane of existence through the use of Jump drive technology, specifically our forcing our way into their universe and trampling all over their civilisation like a bunch of drunkards in a china shop. As can be imagined from this metaphor, we did enough damage to their civilisation that they tracked us back to this dimension and tried to wipe us out, and in response they were exterminated to the point that only a few hundred survivors remained, fed only enough psychic energy to stay alive and produce more Zro as needed for the Union's industry. When the Reapers came and were pushed back their retreat was cut off by releasing several of these that had been planted in their retreat path at the same time, and while effective the Unbidden now stand head and shoulders as one of the three biggest threats to the Galaxy short of the Reapers themselves.
Military
Aside from the individual units mentioned above, the Unbidden use large ships also composed of the same Zro Material that makes up their physical bodies and as a result are extremely delicate. This delicacy is made up for by dint of their shields and sheer numbers but for the most part are unimpressive by Union standards.
Conclusion
It's obvious that the Unbidden would make formidable allies against the Reapers, if such an alliance could be brokered, but it's not known if such a thing is possible, they hate us that much. For now there's at least one willing to work with us. So Progress?