My players decided to try summoning an elder god in order to gain immortality in Call of Cthulhu. Your best ideas for how this will inevitably go horribly wrong?
The bad news: Summoning an entire elder god is hard. They end up summoning one of the innumerable lesser horrors in the elder god's orbit.
The good news: The summoned entity is going to grant them immortality.
The bad news: The players will experience an eternity of never-ending horror and torment.
The good news: Since it's just a (relatively) lesser entity, the players actually have a slim chance of ending this curse, returning to blissful mortality.