Hearthpwn

It's simple. When you use it, you gain the Hero Power of the CURRENT boss you're fighting for the entire run. Of course, it remains in your deck once you defeat a Boss. So, you can use it again! 

What Boss is that? Each boss you play? The final boss of the run?

The card was offered to me as an option pretty early during my first run.

But, since the card text is not very specific, I did not choose it.

I hope I don't sound ultra-stupid.

I've been wanting to do this since getting a Sideshow Spelleater but it never worked until now.