This is a good thought. A man-sized ghoul conglomeration of diminutive or fine creatures. It looks like an ordinary ghoul until up close, when it attacks and splits, covering the victim in tiny little ghouls. They would be easy to kill, sure, but it might take some time to kill 100 of them! Meanwhile, they are working away trying to get those natural 20s to hit you and inflict 1 point of damage but forcing a save versus paralysis.
The only problem with this creature is justifying how they created ghouls so small. In the monster manual, there are no creatures of Diminutive or Fine size, I believe. Tiny creatures like sprites could work, but wouldn't be found in the Underdark.