The silence that announces death. The undying horde. The Hollow Men. Necrontyr. The survivors of the Spring Breeze use many names for them, all the same futile attempt to put a name to and thus weaken their terror. All that matters is they are.
The Harbinger’s Sojourn meetings with them have been (mercifully, perhaps) brief, fighting against their ships and mindless soldiers in the Processional of the Damned. They appear to favour white and gold, and machines, with only two entities showing any signs of self-awareness. Their technology is potent for xenos heresy, and their capital ship was only destroyed by the detonation of a dozen plasma drives and a major warp incursion. Their leader claimed that they could swap bodies with ease if the shells were destroyed; this is thus far unconfirmed.