Quote Originally Posted by radmelon View Post
That's not a fun answer, rather anticlimactic in fact.

I'm just gonna make a huge explosion than summon something high-level.
The proper response to your players should be "Why don't you try it and find out?" followed by an evil grin.
Instead of just a huge explosion, why not have it summon a Living Spell (in this case, a 10d6 lightning bolt) that seeks out the nearest person and attacks them. The Living Spell dissipates one round after it attacks. If the person is killed by the lightning, they immediately become a tainted of their hit dice and class (and the player is ordered to do his utmost to kill his former friends). If the character survives the lightning, they must make a save to avoid becoming a fallen.