Umbra

MATT WARD WILL WRITE THEIR CODEX AND IT WILL BE PURE CHEESE AND MAKE EVERYONE RAEGEE

Ahem.

A minor Xenos race detailed in Xenology, the Umbra are basically black 8-ball creatures with the ability to manipulate darkness & shadow as a weapon, even forming teeth and blades out of the dark spots in your eyes to chew your brain out from the inside.

They are the only thing that can be hurt by imperial guard standard flash light because they are vulnerable to light though and can be killed by shooting them into a corner with torchlight and leaving them nowhere to escape to. (Someone should have told the Ultramarines that, because they are shown getting up close and personal in a fight with them) When Umbra die they psychically scream LINGER into the brains of anyone nearby.

They instinctively collect around warp engines and entrances to the webway, it is believed they are multi-dimensional beings and the fact that we only see an 8-ball is because our minds cannot stretch far enough to visualize them properly.

Apparently they are shards of the Hrud god: Qah (he who LINGERS), and was broken into tiny bits and cast into the material realm by Slaanesh when he/she/it was born and that they congregate around access points to the warp aching to return home (and presumably coalesce).

note: may or may not be the haemonculus in the flask from FMA. Or Leliel from Evangelion for that matter, a spheric multidimensional being which project a tridimensional 'shadow'.