Not long ago, I created an NPC for my level 3 D&D party to encounter. The NPC is a little kitty in a basement beneath an abandoned tower full of magical treasures. As soon as the party began to take any items from the basement, the cat turned on them, swiping its little claws at the ankles of the daring adventurers. Two floating weapons assaulted the party together with the cat to prevent the party from stealing anything. The weapons were relatively easy to dispose of, but the cat just would not die: Every time someone struck a killing blow, the cat just reappeared, bigger and stronger.
By its ninth live, the cat was the size of a ferocious tiger.
Below, the stat block for the guardian cat. Also available as org and pdf. Thanks to Dungeon Dad for the inspiration.