I DM for my kids. Our first dungeon ended with an ogre-mage sending undead after the party.

The heroes made quick work of most of the zombies and skeletons - and critted the ogre-mage so fast that the dungeon was cleared in record time...

And then the party's necromancer (My oldest) decided to raise the ogre-mage. And made them his undead lich-thrall, with instructions to rule the dungeon until the necromancer's return to the region...

Huh.