Fellwinter is gone is what happened to us the 1st time. we were in the same instace and went for one event to the other.
next time we go Fellwinter we loged for the night and next night Fellwinter did not disaper on new instace of Just Beltron.
later redung FC2 we Got a 2nd Fellwinter and used that old Fellwinter in the same instance and POOF gone. -lol
So to keep Fellwinter get a new Instance beteen that mob that drops it and the Door of Beltron.