Xorians are one curious alien race which was exact discovery is unknown but very first Xorian infection was reported to have occured at 2,000 years ago according one Xorian infested planet's database which was miraclously still functional. Xorians have been most aggressive and volatile faction across all other factions, there has been (suicidal) attempts to negotiate with the race but with Xoriens you cannot negotiate with for several reasons; most Xorians are instinct driven, no more than animals in basic inteligence. Then those who show inteligence consider themselves above all other races in galaxy and thus 'negotiations' which is based on talks between equals is unknown notion for them. Only form of negotiation is how much other makes it easy to invade and be assimilated by Xorians to join the brood swarms. If done willingly they do not kill anyone nor destroy buildings. But that's it really.
Xorians as race has been lenghty studies as main reason is that Xorians rapidly evolve and change based on environment and also species they assimilate to their ranks. They can assimilate any
biological race as part of the brood, using any ideal and best genetical aspects of any race, remove any faulty ones to replace with their own superior genetics. Assimilation process is mutagenetic process where they forcibly inject their warping biomass upon vessel, let the biomass start infect and warp biological matter in the victim. The rapidity of the change depend on frequency biomass is injected and also individual resistance to warping changes but so far there has been no case of immunity.
Also there is process where victims have been dipped down to acidic ooze where their entire bodies are melted down to spare parts and then rebuild anew in this acidic breeding pits where they are not merely improved but created something anew from new cloth. These breeding pits have resulted to most individual and powerful Xorian members which most succesful ones can ascend to heights of Overlords, Xorians who govern over other Xorians through psychic link.
Xorians may lack in industrial matters or technology and are incapable using technology, but in biological warfare and manner to adapt to any situation or environment is truly frigthening as is their ability to convert any soldiers from any factions as their own slaves or soldiers who provide inteligence to the host. Also let's not forget their psychic powers which covers many aspects what traditional technology could produce (like long distance space communication or space travel and navigation). Xorians are universally labelled as faction of 'invading alien monsters' who raid worlds for biomass and infect and corrupt worlds to become their own breeding grounds. What purpose? None other than just spread and consume more biomass, evolve more and grow more powerful. Xorians are obsessed in evolving, growing stronger and consuming biomass. In short species or foes which they do not plan convert they plainly eat and gain properties of the consumed being.
This also results in one their weaknesses if any; if more than one overlord attacks a planet they often fight against each other as their shared enemies. As Xorians can attack each other and consume biomass from other Xorian to get most benefit and greatest boost in evolution and growth in power. So two overlords who run their own xorian hosts may seek evolve their own host by consuming other host. So in this way Xorians are not united at all and act more in individual interest and grasps for power than by any united council or by single leader. Which also is their strenght; as if there would be council or single leader, the Xorian host could be crippled with single tactical strike. But you cannot, Xorians spread, breed rapidly and evolve rapidly and are hard to keep track off.
So for these reason most cases factions could hold temporal cease fire in case of Xorian invasion.....as no faction alone can face the Xorian threat alone and survive.