With Deathwatch just around the corner, I find myself pondering what kinds of xenos to throw at a party of veteran Astartes other than the standard GW fair. So I thought I'd start a topic to share our ideas on such a topic. Let us know what you've got brewing in that brain of yours.
Here's the short version of an idea I've been working on:
Species name: Yzsrid
Physiology: Amorphous with no localized organs or skeletal structure; capable of taking any roughly human-sized shape, both living creature and inanimate object; incredibly resilient with hyper-regenerative abilities and exceptional physical strength; inborn psychic capabilities of exceptional level; reproduce by implanting a portion of their genetic material (or whatever it is) within a living host which grows, not unlike a cancerous tumor, until it eventually subsumes the host completely; share a racial memory that is passed from generation to generation so that newborn Yzsrid have all of the knowledge and abilities of their elders upon full emergence, and thus have no need for parenting or protection; natural form (form assumed after death) is that of a viscous opalescent slime lacking in any discernable features whatsoever
Culture: As a species, the Yzsrid are devoted servants of Tzeentch. It is theorized that their bizarre physical traits may be gifts from their patron, and that they might have once been a more typical humanoid species. They are universally deceptive and manipulative creatures which use both their innate shapechanging abilities and the sorcerous gifts of their lord to control and dominate those around them. Devout servants of the Changer of Ways, they are ever serving him and his incomprehensible schemes, existing as a deadly yet unseen parasite within the flesh of the Imperium.
Implications: Only one such creature has been knowingly encountered and interrogated by the Inquisition. The little information that was attained in that event (presented above) is, needless to say, highly disturbing to the Ordo Xenos. It is suspected that there are likely many more such abominations at large within the Imperium.