Someday, and that day may never come, I will call upon you to do a service for me.
A Hag is a manipulative bastard. It knows that it will win in the end, so it can give a little bit up front. It can be the godfather clause, never specifying what it will want... and then it comes to claim everything. Or just a tiny thing. Or the wife of the hero. But whatever it claims, it will destroy the luck and life of the hero.
You promised me, and now I have returned to claim what you owe me.
Oh Midas, be careful what you wish for!
But it musn't be the hag that returns that brings the downfall. What if the hero himself brings his own downfall? Best even with the very gift he got? Just be very very careful at picking up each and every chance to exploit the deal... and then have it executed to the letter - and usually that alone brings a lot of downfall.
Oh, the dagger that kills everything it cuts... it just happens to be in the kitchen and how clumsy the hero's fiancé is when cutting onions...
What was the story about Midas and turning everything he touches into gold again?
And then we have gifts that are just too good to give up. Too good to share. Too good for others to know about! And yes, these gifts make really paranoid.
Pesky cleric that found out about this ring of invisibility. Once you were my friend, now you are just a corpse. A pitty I had to stab you a couple times to keep the ring. What, the Paladin doesn't like that the cleric 'ran off to the woods' during my guard? I will have to rectify that...