Latter Day Sinner

The most fun monster I ever created for dungeons and dragons was a Smurf. They had one hit point and could only be killed by one hit point of strangulation. Two hit points popped them in half and the two halves grew into two new Smurfs. All extra damage had a similar effect. 10 points of damage meant 10 new Smurfs. They danced around the players singing which eventually drove them insane. The last time I used them the players took a bag of Smurfs and threw it at a red dragon. The dragon incinerated the bag making thousands of pieces of Smurf ash which regrew and eventually suffocated the dragon, buried all his treasure, and filled the entire dungeon. The players never wanted to see another Smurf in all our following campaigns.