You unlock Pompous Thespian by defeating the first boss in the Opera, Julianne. It’s another fun battle and carries on the storyline, and on normal mode it’s way too easy so don’t worry. It’s the second week of the adventure, and the second wing to open (not including the prologue).
I completed it easily with a cheap Zoolock deck. All you have to do is kill the 4/2 minion on board to destroy the immunity from Julianne. You will be able to do this with basic cards on just about any class, it’s the heroic mode that is super hard, but you don’t need to complete that for the cards.
The card reads: Taunt.
A simple neutral card. Probably more of interest to arena players than constructed.