It’s been teased since that bombshell of a trailer
http://io9.gizmodo.com/relive-the-tra...
Yes, just as many fans had guessed, the “Old Master” Ezra meets on Malachor in the season finale is none other than Darth Maul, who has so far survived being sliced in half at the end of The Phantom Menace, and then getting his butt force-kicked by Palpatine in Clone Wars. You’ve got to admire the dude’s persistence, at least, and that he’s still kicking around five years before the events of A New Hope.
Maul’s presence in the finale—born out of the realization that the character could fight Darth Vader, according to producer Dave Filoni—is to act as a potential mentor in the Dark Side for Ezra:
We’ve taken all season long to educate viewers that maybe haven’t seen Ahsoka before about what her relationship to Vader is. But then we had this other thing where we all liked Maul… Maul could be a fantastic foil for Ezra, a counterpointed dark-side mentor. He’s not necessarily the dark-side Emperor Sith...
He’s this person who has been on the dark-side, that has been thrown out repeatedly from the Emperor’s hierarchy but that doesn’t mean he doesn’t have a lot of the same tendencies as the Emperor and have the same desire for power. It became a really interesting story to tell, and a challenge, because at face value, Maul isn’t a guy you look at and go, ‘I trust you, devil-looking guy with horns on your head.’ So we had to paint an interesting picture here to get this kid to perhaps believe in him.
The direction Clone Wars—and now Rebels—took Maul in has been fascinating to watch unfold, especially for a character whose movie prescence consisted solely of had three lines and a swishy lightsaber. The thought of an older, sadder Maul languishing and attempting to draw Ezra to the Dark Side certainly makes for an interesting foil to all the worrying we’ll be doing about Ahsoka in the finale next week. Head on over to the link below to read more, but first, here’s a clip of Maul’s Rebels return: