D&D games, in general, do not come with nonremovable death curses. D&D is meant to be a heroic game where characters overcome problems, so all its curses conecome with built-in ways to overcome. Geas is an excellent example: it does exactly what you want, but it comes with an escape hatch and that makes it unsuitable for your adventure.
I'm worried that this question is going to attract a lot of unsuitable answers: answers which require a completely unrealistic expenditure of resources on the part of the captors and still don't fit your criterion of being inescapable. If (as I suspect) you're looking for a solution that preserves verisimilitude, your campaign will work much better if you put on your houserule hat and say: "this geas spell was sealed by a cleric of the dark god Besz-ul-Qoma, so it can only be unsealed by another cleric of that god, and Besz-ul-Qoma is Lawful Evil so none of its clerics will help you."
If you must have a RAW solution, you might try importing the 3.5e version of the spell, which can only be removed by someone two levels higher than the caster.
