Kind of old info, but it isnt posted here yet!
Fellwinter is dropped 100% of the time from Harfange the Black. It's used to unlock Beltron, The Shade King's door. The item is unequippable (of course). The item will disappear out of your inventory when you click to open Beltron's door IF you attempt to kill Beltron in the same night that you kill one of the other bosses. If you go back to try Beltron only (like a new instance with none of the other raid mobs up and all the trash at the entrance repopped), then Fellwinter will not be consumed when opening Beltron's door, though it's still required.
I hope that helps some. :)
Edited, May 13th 2008 6:02am by Aiwendill