I'm curious to know if anyone has tried this yet and what boss it features. I'm also curious to know what kind of loot can drop in this stage or if it adds any.

yep its pretty snazzy kinda forgot to put in the drops but i'll add them in a bit 
 http://www.giantbomb.com/castlevania-harmony-of-despair/61-31137/chapter-7-thoughts-boss-guide-spoiler-ridden/35-453926/#1

It's pretty shitty.  It's just one new map with 3 new enemies and 4 new items (all of which suck) for 1/3 the price of the game.  I love the game itself, but this DLC is worth maybe $2.  And that would be pushing it.

There's one new regular enemy (Mummy) and one new boss (Astarte). There aren't many worthwhile new drops, although Charlotte can absorb Delta Spark from Astarte, and Soma can get souls from Astarte and Mummy. I'd say that the map is worth the money, but I'm the sort of person who's gotten far more replay value from HD for the money. I've just played the game a lot more than any other game on Xbox 360 (even games that I paid $60 for), so paying $5 for a new level that changes things up a bit is worth it for me. 
 
For the record, Astarte requires significantly different tactics when compared to any other boss, and she pretty much neuters Alucards and Somas who spam Valmanway/Yasutsuna. She has some excellent drops, including Winged Boots (Hard), as well as Lapiste and Pneuma (any difficulty) for Shanoa. I've heard that she also drops Valmanway (Hard).

From what I understand, it's a level from Portrait of Ruin.