The Umbra are an intelligent, nonhumanoid alien species of the galaxy that is intensely hostile to other forms of life and have represented a threat to the Imperium of Man on numerous occasions. Very little is currently understood about this hostile and enigmatic species of xenos.
Anatomy and Physiology
An Umbra appears to be a black sphere to the naked eye, but it possesses the capacity to manipulate shadows as weapons. The Umbra were known to be related in some way to the Warp and were often sighted near starships' Warp-Drives.
With an ability to manipulate the shadows, the Umbra display a direct aversion to light. Experiments performed by Magos Biologis Sharle Darvus demonstrated that containment within an area lacking any shadows was sufficient to euthanise the creatures.
One theory put forward by Calculus-Logi Byrr is that the physical form of the Umbra is but a small reflection of the whole creature, with the rest existing in another dimension, most likely the Warp. This could help account for the creature's ability to manipulate and control shadows.
Upon the captured Umbra's death, both Magos Darvus and an accompanying Inquisitor experienced "intense cranial pain, a flickering succession of mental images broadcast by the creature, including a humanoid figure splintering apart, the image of a Warp Storm and deep space, and a single word, 'Linger'". This seems to denote that the Umbra possess some form of psychic ability, even if it is extremely limited.
A questionable text penned by a self-confessed Heretic speaks of a "war in the heavens between star devils and old gods," likely a reference to the C'tan and the Old Ones and their War in Heaven millions of Terran years ago. After all the old gods were killed, one survived and hid within the Warp. Then she was "born in the longears' brain," and the Chaos God Slaanesh, "chops him a million times and kicks the shards out into the cold. To Linger, she says to linger like always." An incredible account, despite its lack of credibility or comprehensibility.
These descriptions could be considered consistent with the visions experienced by Magos Darvus. This would suggest that the Umbra are a fragment of a greater entity, perhaps one of the C'tan or some other unknown Warp entity. Noting that they are often found attached to Warp engines or in areas suspected of being entrances to the Aeldari Webway may denote an attempt to return to the Warp.
Perhaps these stories relate to the battle between Slaanesh and Khorne over the fallen Eldar War God Khaine, as Aeldari myth claims that he was split into a thousand pieces which were scattered across the galaxy. As ever, all the information available about the Umbra is mere speculation. In their spherical form these beings are poisoned by the light, their only known vulnerability.
- Xenology (Background Book), pp. 35-39