The go-to monkey paw scenario could work, where they receive immortality, but have to serve the summoned entity forever as deformed (and perhaps always hurting) “things”? However, that is maybe a bit too predictable. What if they mess up the spell and instead mark themselves for sacrifice?
They slowly realise this due to a symbol appearing on them, which could slowly spread further on their body (maybe hurting too). You could make it more interesting by pitting them against one another by suggesting that only a few have to die for the sacrifice to be complete. If their characters are not that close to one another, it could lead to some interesting decisions on their part, haha.
The process could span a week or more, where their body gets engulfed more and more by the mark. They could use this time to review what went wrong in the summoning process, and how to potentially reverse it. I’ll leave those details to you, if you’re interested in pursuing this idea :D
Hmm, very interesting! This could definitely go further into the first scenario. Perhaps the summoner is now a “puppet” for the entity, and is trying to trick the more resilient player characters to succumb to the entity’s control. Perhaps the first ritual weeded off the weaker candidates, and now the entity is preparing to give them the full course.
I’m curious what you’re going to go with :D