Huge story DLC was added and Dahl scene. Plus art change and probably balance changes. This is all as far as I know.
There's a new Dahl scene? Beyond the original one where he
forcibly kisses Celine?
To answer the previous guy's question - any balance changes were extremely minor. More just bug removals. Julia infinite shield stacking was fixed, so Knight's not quite as good anymore.
Besides that almost nothing different, a couple of the scroll abilities were tweaked a bit.
The big "dlc" becomes available after you beat the main game. It is a "what if" scenario that takes place following the bad ending. Gameplay in the dlc is exactly the same tho EXCEPT
The one major change gameplay-wise would be the addition of a lot (like 7-10 or so) "active" abilities that you use during battle which are chosen among your 10 "scrolls". Thing is, I didn't even touch them in the main game because I hate needing to do something like that when I want to play auto chess.
BUT the main difference or the "dlc" campaign is that those active abilities get majorly buffed. So you pretty much have to use them
Oh also endless mode got harder difficulty levels and stuff up to 5. It's actually really hard on the top difficulty level and you can't get past 20 waves or so without going for super OP specific buolds