Atlantis was his chosen domain (8)
I believe the answer is:
poseidon
'atlantis was his chosen domain' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for poseidon that I've seen before include "Poisoned the Greek god of the sea" , "Controller of the main" , "being able to create waves?" , "Greek sea-god" , "underwater missile" .)