Maybe I missed it, but how did you as GM plan to deal with this? After all, you set up the encounter.
Absolutely. It's important to know what you're really throwing at the characters. Having said that, I think that this is also partially the result of poor game design. It says they are stunned until the regain at least 1 point of Intelligence, but doesn't give mechanics on how to do that. Compound that to the fact that it either causes no Intelligence damage or you're reduced to 0.
I haven't dealt with the intellect devourer directly, but I'd change the attack to 1d4 Intelligence lost per failed save. In addition, in my campaign ability damage is regained by making a 'death' save at the end of a long rest. Three non-consecutive losses before 3 non-consecutive successes means 1 point of the loss is permanent.
It takes a long time to recover ability damage, but I also reduce the amount of ability damage a creature can cause in a single attack to no more than 1d4, if that. Creatures that can cause ability damage should be terrifying, and the threat of a permanent loss should be significant. But recovering from being drained to '0' is much tougher.
So now that you're in this situation, how do you get out? Well, first and foremost you can't exclude somebody from the game for an extended period of time. That's entirely against the point of the game itself, and more importantly is taking the 'that's what the dice said' thing way too far.
A mercy killing is not going to be enough to lose class abilities. A single act is (very) rarely enough. That doesn't mean it's not evil, but it's really a pattern of behavior that matters more than a single act. Think of Anakin Skywalker and the path to the dark side if you will.
The game doesn't actually have any rules at this point about losing class abilities, although in the DMG it talks about atonement for an Oathbreaker Paladin. Regardless, it would have to be much more than this for a God to take notice in my book.
But quite frankly, I think the mercy killing is a red herring.
If the cleric and paladin aren't doing everything in their power to help a fallen comrade, then they probably should lose their class abilities. I'm sorry, but this is a situation where the quest is a failure, you retreat, recover and return. There shouldn't be a moment's hesitation or debate.
Think about it, you're on some grand quest, miles from civilization and your friend suffers a traumatic head injury and is unable to do anything on their own - and your first reaction isn't, 'we've got to get them help, and now?'
Huh?
HELLO! Cleric and Paladin critical failure on common sense and compassion.
In the meantime she absolutely should be able to roll up a second character. It could be anything you want, either one that is helpful, or if you want to play off of the misfortune, a hindrance. A defenseless and unarmed slave escaped from duergar. Now the clueless and heartless cleric and paladin have somebody who can speak to save and get to the surface unharmed.
If they can't turn back for whatever reason (they don't have a way to go back because it's blocked, or the greater good demands they continue) then their primary focus should still be on their fallen comrade, and the player should still be allowed to play another character. There is nothing in the rules that would even suggest that a player cannot have a second character, especially when their primary one is incapacitated indefinitely.
In the campaign I'm starting up, everybody is rolling up 3 characters to have some ready for future sessions. Granted, I'm designing the campaign for a public game at a store where I won't know from week to week who might show up. So I'm expecting drop-ins and no-shows and accommodating that. But there are an endless number of scenarios to come across a new character in the Underdark.
Ilbranteloth