Established relationship, game timeline preferred. For whatever reason, Noct is forced to bargain with [insert antagonist here]; their price is a night with Ignis, and after wrestling with the decision Noct decides, miserably, that it has to be paid.

Possible scenarios:
-- they're trapped and overwhelmed in a dungeon and a hunter shows up and offers to help them out
-- Emperor Aldercapt says he might be more amenable to calling off hostilities with proper incentive
-- Prompto/Gladio/Ignis himself is dying and Ardyn suddenly shows up with a hi-potion
-- Caligo or whoever offers to stop destroying Altissia if Noct cooperates

I want Noct to be the only one making the decision (i.e. Ignis doesn't offer himself up), and I'd love it if he had to be the one to tell Ignis, too. What I'm really going for is maximum angst and betrayal; you don't have to write the sex itself, but I would absolutely love to read it if you're so inclined.

Bonuses:
+ Noct tries to offer himself up instead, but the antagonist wants to make Noct suffer even worse
+ the help is rendered before the deal is fulfilled, so the entire time they're being helped it's just apprehensive dread about the impending event
+ Noct insists on watching to make sure the other guy doesn't hurt/take advantage of Ignis
+ the other guy makes sure to make it good for Ignis, too (maybe even mocks them for Ignis coming while a stranger fucks him)