Jason Blood is a slim, mature man with a gray streak through his hair, speaks with an English accent, and is able to call up Etrigan from a place of fire (presumably Hell) by saying the following rhyme:
- Gone! Gone! Form of man,
Arise the demon Etrigan!
When this happens, Blood is replaced by the demon who is then free to operate in the world. We never see the reverse action. He must have some familarity with magic as he is able to levitate in the air while he reads his arcane books in his house probably somewhere in Gotham City. Jason Blood is known by Batman and, as he is present at the Justice League's Christmas party, he is presumably a member.
- Etrigan at Wikipedia.