Scarlet's Game Room
I know of 3 creatures that drop Curse. There could be more, I don't have the scoop on ALL the bosses out there yet. I been trying to get Alex to do my work for me - uh, I mean HELP me, but I haven't had much luck yet.
Barkingham Palace, Marleybone
Loremaster's Chamber, Dragonspyre
Noxious Gas Golem
Main Hall, Marleybone