Included in the pack are "the Disco Inferno costume, Stone of Wrath relic, and a prequel level featuring new enemies, puzzles, and storyline!"
PS3 users that purchased the game new should already have a redeemable code that will net them the prequel level, which was distributed in the PlayStation 3 exclusive "Divine Edition". The next DLC pack, set for an April 29 release, will include co-op multiplayer and a gameplay editor.
Thanks to Chris Faylor, who spotted this DLC on his PlayStation 3 while downloading the latest episode of The Tester.
I have it and beat it already. It's pretty short and the new enemy is nothing special. I didn't really like that you couldn't use a saved game and they just start you with full holy / unholy trees and no relics.