Nagga's Hill
Iron Islands Response: After you win an unopposed challenge, kneel Nagga's Hill to choose a non-unique, non-event card in the losing player's discard pile. Put that card into play under your control. At the end of the phase, discard that card if it is still in play.
Can my opponent choose FK and attach to my character to kill him?
OK, so I guess this happened yesterday after I was gone. 
Why wouldn't it work? All you need to know is:
- Is FK unique? No, it isn't.
- Is FK an event? No, it isn't.
- Is it in your opponent's discard pile? Clearly, or the whole question wouldn't come up.
So, play restrictions are clearly met. Why wouldn't it work with Nagga's Hill?
Don't worry, he did the same thing to me 
Sorry it is still something which left from my Legend of Five Rings CCG where to put something into play you must always pay cost. Anyway broken for me.
berto said:
Anyway broken for me.
Well, it was done to me yesterday, and I won right afterwards, so I'm not sure about broken.
Oh, BTW, of course I was wrong about the Hatchling's Feast thing.
Sorry, other guys, I'll stop making this a private conversation now.
berto said:
If it helps, technically you are paying the cost - of Nagga's Hill.