Return to Its Master did not play correctly at site 6 (and yes, I know it's complicated):
http://www.gempukku.com/gemp-lotr/game.html?replayId=ramolnar$8cwpjntdfldxxzi5
I waited until at the end of the fierce skirmish phase with Nelya. The Ring was on, because Vrado had converted a wound to a burden. I should have been able to assign a Nazgul to Frodo, but nothing happened.
Hmm, there are some fundamental problems with the card:
1. If you use it at the end of fierce skirmish, only Fierce minions can be assigned using the card, as you can assign only Fierce minions to skirmish, during fierce skirmishes.
2. You can't use it on a Nazgul or Ring-bearer that is in skirmish you are responding to being ended, as it becomes unassigned only after all responses to end of skirmish have been resolved. Therefore due to rules about assigning, you can't make an assignment that does not create an assignment between two unassigned characters.
This is the reason why the game did not allow you to choose a Nazgul to assign to Ring-bearer. Only one of your Nazguls had Fierce, and it was a Nazgul that was finishing it's skirmish, so was still assigned against a character, at the point, when you used
Return to Its Master.
So, from rules point of view, the card works perfectly, though maybe the card should be changed how it works, to achieve what the intention of the card was. If so, we need an agreement how it should be changed.