It looks like that's a question we'll need to work out. I didn't write the document that updates and explains our rules for set-aside abilities but I think this statement explains the intention behind it:
If more than one card is set aside with a specified duration, can be prematurely returned, and if one of those cards is returned from the set-aside area prematurely, then the other card(s) set aside with it immediately return to their owner's territories with no further benefits gained.
This will need to be adjusted to include the possibility of characters leaving set-aside early for other reasons, like being discarded by Darius' Decree. Based on what I believe to be the intension behind this statement I actually think that your Nergalsharezer would be returned to play early, not left in set-aside indefinitely.
However, currently, until that's ironed out, I would rule that he stays set-aside until the full duration is reached and returns as normal. I don't find any statements pertaining to this in the current REG.
