Istbor
Dances with Gnolls
Well, you see, there's this army of undead that's slowly growing on the outskirts of a kingdom, and adventurers were sent in to figure it out. Each group that was sent in never came back* (they were all killed by the CL + Sim), until a high enough level group shows up.
No one has ever figured out his goals, but they do know he indiscriminately kills anyone who comes near to build up his army of undead, so he must he taken down.
*The rare survivor has told you of the equipment and tactics the enemy uses.
From where were these corpses and piles of bones, that this undead army comes from? Plus, awful. You have to command them to do anything. If you need to quickly change what you want them to do, you have to be within like 60 feet of them, or at least the ones you want to do a different thing. And undead army without any strong willed undead is not a difficult task for a group of high level adventurers, let alone a whole kingdom.
Again. This tells me nothing of what this forgotten man even wants. Or who he is. Apparently he is moving from graveyard to graveyard. So it is easy enough to set the stage for his defeat. Stake out and prep the largest nearby site where he might be able to spend hours raising more dead, and eliminate him. Or, don't. Remove the army. If it is that vast, there is no way he can protect its entirety. Strike the flanks, as he responds, strike where he left.
Out of context, the spell user seems strong, but within an actual game, where scouting, information gathering and smart tactics would exist, he is made vulnerable by his own perceived strengths.