Chruul

The Chruul is one of the Alien Factions in Halcyon 6.

History
These horrific aliens are the main antagonists of the game.

They initially appear from portals and quickly overwhelm the forces of all known factions, basically destroying most of the Federation.

Their home world seems to be in another dimension, where the Mothership is located.

The Chruul pilot massive biovessels and seem to attack and destroy species indiscriminately. They're highly intelligent however, possessing advanced tactical knowledge, powerful weaponry and psionic powers that enable them to mind control other species.

It is revealed late game through a set of special missions that the Chruul were once a regular space faring species. One day they came upon a massive space station and decided to not pass the opportunity for some free Forerunner technology. The station corrupted the Chruul however, turning them into obedient soldiers that followed the will of this new Mothership. Some of them escaped the mind control and hid deep underground to avoid their zombified comrades.