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?

This particular 'elder god' is more of a mantle, you see. This entity does wield great powers, but is also beholden to a horrible and cursed collection of duties. During the summoning, they accidentally, somehow, kill this elder god; due to the nature of the summoning they must now assume his mantle and fulfill his obligations, with all that entails, lest the cosmos fall. A Satan Clause, if you will.