Dross is one of the new stages of the Binding of Isaac added in Repentance, which can be accessed by taking an alternative path.

However, the place is so disgusting (and has annoying bosses) that I always skip it. It's not even just the challenge because sometimes I fight Mega Satan / Delirium / Hush for fun but I never like going to Dross, and I don't think anyone likes it outside perhaps Edmund.

Downpour (the other alternate stage) thou, it's nice. Love the music.