The Praetors are the alien masters and are only encountered in the very late game. They reside aboard Battleship UFOs and inside large alien bases. They expel an aura of dread, which reduces the TUs of anyone affected significantly.
Praetors are alien leaders. They are the most powerful Alien race in the game. They have a high pool of health, a lot of action points, good accuracy, high reflexes, high bravery, which makes them impossible to suppress, and they are the most powerful psionic wielder in the game. They exhibit a natural aura of "dread" which will randomly reduce Xenonauts soldiers' action points at the start of a turn, as well as being capable of Mind Controlling humans.
Praetors are a key component to the tech tree, and one needs to be captured alive in order to advance the plot. Praetors are only found onboard Battleships and Large Alien Bases. Praetors will always be found inside a UFOs bridge or a base's command center. In addition to their impressive combat skills, they will also be heavily guarded by Elite units and a Leader. This makes getting to a Praetor extremely difficult, let alone capturing one alive.
Praetors are resistant, but not immune to Stun Gas, so either Stun Batons or melee attacks are advised. Since Praetors do not move from their location, it will make it more effective to set up several soldiers near the Praetor to stun it.
The Praetor is a frail, withered creature standing 180 cm (5 foot 10 inches) tall...well, I say "standing", but field reports suggest this creature is an exceptionally powerful psionic entity capable of levitating itself through the air using only the power of thought. Tissue analysis suggests these creatures are tens of millions of years older than any of the other alien species we have encountered; they are almost certainly the extraterrestrial command caste.
The white armor these creatures wear is lightweight and presumably largely ceremonial in nature, offering little actual protection. The body beneath is underdeveloped, with a brittle skeleton and internal organs that are barely functional - indeed the creature's circulatory system is synthetic, driven by a powerful "heart" implanted between the shoulder blades. This supplies nutrients to the alien's prodigious brain, which is almost twice the size of the human equivalent and contains several extremely complex structures packed with respiratory cells that metabolize Alenium nanoparticles from the bloodstream. This would produce vast amounts of energy; we assume it is this that allows them to manifest their psionic abilities.
This has powerful implications. These creatures have clearly surpassed the limits of their biology, engineering their brains and bodies to gain immense psionic powers. They long since passed the point where they can sustain themselves without their technology; even at rest, their brains consume more energy than is generated by the average nuclear power plant. Only regular infusions of Alenium nanoparticles can stave off fatal brain starvation. Has their machinery released them, or imprisoned them? Both, perhaps.
Anyway, it goes without saying that the psionic strength of these creatures makes them extremely dangerous on the battlefield. You should also expect them to be difficult to kill, as I strongly suspect that their psionic powers grant them far more protection than conventional armor ever would.
We finally have our answers, Commander. These creatures are biologically immortal, immune to aging - but certainly not to violent injury. Our captive is evidently terrified by the possibility of death and proved exceptionally compliant during the interrogation.
The Praetors are an ancient race that developed hyperspace technology millions of years ago. Their history was unremarkable until they encountered the Caesans. The Caesans of old had settled their home system but did not possess the hyperdrive technology they needed to travel beyond it. What they did possess was the power of telepathy, along with several other latent psionic abilities. It proved their undoing; craving the powers for themselves, the Praetors subjugated the Caesans and studied them until they identified the genetic material responsible.
They improved this genetic material and granted themselves extraordinary psionic powers. They became a race of intergalactic tyrants, seeking out and enslaving sentient life in an endless pursuit of 'improvements' (e.g. the biological immortality extracted from the Sebillians). The useful species were retained, genetically re-sequenced to ensure submission and servitude.
This is the fate planned for us. Our enemies toy with us - this invasion is just a test. The High Praetor measures our progress from the bridge of the enormous alien mothership orbiting our planet. Without him, the fleet would collapse...but then the extraterrestrials would simply jump in a new fleet. There would be no end. At best, we would have a choice between extinction or subjugation. Neither much appeals.
The rules must change, Commander. I have an idea...just give me a little time to develop it.