I think Sifo Dyas is Darth Maul. He was on the Jedi council, like Anakin, "working" with Palpatine / Sideous, then during the events of the Phantom Menace, they decided it was time to "reveal themselves to the Jedi. Ordered the clones, transformed to Darth Maul, got a cool looking face tattoo so no-one would recognise him and there ya go!