A Faustian bargain. A deal with the devil.
What is it, precisely, that makes a Fasustian bargain Fasustian? Any bargain involves exchanging X for Y, and that’s cool. But a Faustian bargain is more than simply an unfair or ill-considered bargain. It’s more than simply that X was actually worth more than Y. It’s that X was the thing that gave Y its value. That when you surrender X to get Y, Y becomes worthless because you have surrendered X.
Wilde (I think) does this in “The Gift”. The classic is the exchange of your soul for power. Once you have surrendered your soul, any power you might wield you only wield at the command of the one to whom your soul now belongs.
Awesome as Y might be, if you surrender the entire point of having Y in order to get it, then you have made a bad bargain.