[Tiavals]

While I may not be more experienced than you, here's my thoughts:

Ermor was allowed in the roster despite the suggestion that it's too powerful in a non-diplo game.

It's easier to dispel spells than cast them. (You can turn gems to astral pearls much easier than to other gems[except if the enemy is cunning and casts it weak at first so someone will use all their gems for a dispel, then cast it strong afterwards, but with a high base cost, not necessarily viable])

Based on these two things, I'd say there's nothing wrong with casting Utterdark. Then again, I'm not totally sure what it does as I've never been in a game where it has been cast. Some weak shades attack everywhere, income plummets, darkness in every combat? Sure, it's annoying, but it's a level 9 spell that requires plenty of death magic and gems. If you've been hoarding gems for the entire game waiting to cast it, then you've crippled your game up until now. If you haven't, do you really have enough death gems so that it won't be dispelled? I suppose you have had Well of Misery going on for dozens of turns.

Anyway, that's just my thoughts, I'm sure others will have their owns to add.