Cleon
Legend
I like the suggested Spell Reversal.
I'm not sure about the save or die for 5 HD creatures (even in a group). How about "caster gains a negative level" (or a random number of negative levels) instead?
A cockatrice or basilisk has 5 HD and is effectively "save or die" (albeit petrification instead of literal death).
If you really don't fancy save or die, I'd prefer a large amount of damage (like the harm spell's 10 hp/level) over negative levels, since (a) it's more likely to kill a spellcaster, and (b) negative levels are so damn fiddly.