There are some massive spoilers and teases in this Star Wars Rebels mid-season trailer. The rest of season 2 will feature several shocking returns from Star Wars: The Clone Wars, a crossguard lightsaber, and many more huge surprises. Do not watch or read further if you don't want any spoilers and want to watch completely fresh - but nothing in this trailer will do anything but get you incredibly hyped.
The trailer features, among its shocking returns, the lost Sith Lord Darth Maul. At the end of the Clone Wars, Darth Sidious said he still had plans for Maul, and it appears he may be the guardian of some sort of Sith Temple.
The secrets of the Sith will be heavily delved into in this second half of the season, something that has never been done before in a canon product from Lucasfilm. They're borrowing some things from the old Expanded Universe, though, with a Sith Holocron, and an image of the Old Republic's Jedi Order flag.
We also see visions of Anakin Skywalker, and it seems to spell out that Ahsoka knows who Vader is. Yoda.
It's not all looks back, though, as the crossguard lightsaber, a design seen later in the timeline in Star Wars: The Force Awakens is seen, amongst some other possible connections.
Look for more analysis, and more Star Wars Rebels news, later today.