Atahsaia are killing machines, pure and simple. They cannot easily assume a human form, their appetites are voracious, and they know nothing of kindness or pity. Except for Queen Ananasa, no one actually knows what their purpose is in her grand scheme.
Once they've hatched — usually devouring any and all of their siblings, so that only the strongest survives — they are taken to the Deep Umbra where they are sheltered by powerful Spider-spirits or even elder Ananasi. There they are allowed to mature, and using powerful rituals known only to a few carefully selected Damhàn, they are frozen in Amber, preserved for when they are needed.
Ananasa herself is virtually the only creature these nightmares will communicate with. To all others, they are hostile to a fault. Some Ananasi speculate that when the time has come for the Mother-Queen to make her final moves in the elaborate game she plays to usurp the Triat, the Atahsaia will be her chosen warriors against any and all minions of the greater powers in the universe.