Dementians are an extremely violent race. Surprisingly they have not joined the Chaos Legions, although this only seems to be because they don't know about them yet. Instead they serve the Crux, alongside the Lunacites and Frenzish. -- Vacotor Database, Terran translation
Dementians are mute sapients born without vocal chords. They are juggernauts that silently respect their allies (usually) but almost instinctually kill species perceived weaker than them without a single thought to it being a potentially wrong action. A Dementian having a bird land on them will crush it to death in their palm with as much feeling and thought as brushing dust off of one's shoulder, and they appear completely incapable of comprehending how this could be considered anything even worth a second thought.
Interestingly enough, they aren't members of the Chaos Legions, but rather the Crux, though this is only because the Crux found them first. Due to their violent nature, the Crux must utilize them for combat as much as possible to prevent them from getting "restless;" though given the nature of the Crux, this is rarely an issue.