God of terror.
In some versions of the myth, he was the fear and terror that was brought on by war. More of an entity than a God, he fell into a category very similarly to the rest of the primordial Gods. While some versions hinted they might be the sons of Nyx, others mentioned that he and his brother, Phobos were sons of Ares and Aphrodite.
Deimos gots his hands into everything with Phobos and often helped his parents in times of war to create even more chaos. Essentially he and his brother were those sneaky twins that made a mess of things even after you told them not to touch anything.
Deimos and Phobos were, unsurprisingly, worshipped primarily in Sparta.