Originally Posted by
Emperor Tippy
Um, this is standard practice in my high level campaigns.
There is a reason that entire (20+ game) campaigns are devoted to the task of killing (permanently) a single level 20 wizard. It involves doing favors for gods, finding lost artifacts, occasionally messing with time, and exploring the planes; all of that to even get a chance to actually fight the BBEG (not win, just fight him). And that's with an entire party of similarly optimized characters (including both arcane and divine full casters).
And it is still DM fiat which gives them that chance.