Sentient being which was encased within an Ancient containment vessel for approximately ten thousand years. The question of whether or not is intelligent is still a matter of debate, though Dr. Rodney McKay suspected one might have greater luck communicating with a great white shark.
The creature shares identical physical traits to the half-ascended Anubis, being of an oily black color with a less-than-vaporous consistency. This suggests that the shadow creature was also trapped in the same "nether universe" as Anubis, and also why it failed to communicate -- the Ancients perhaps attempted to ascend a lower life form. It is also equally possible that the Ancients located this creature and captured it for study.
Hide and Seek - Jinto, attempting to find his way back to his part of the city, releases the shadow creature from its confines within an Ancient containment vessel.