I saw this on the official forums and thought it was fun, so I'm bringing it here as well.
Pick a champ and give it an ultimate ultimate spell unlocked at level 18.
Darius - Stuns all champs, both enemies and allies, for 5 sec. All enemy champs are brought to 800 health and applied with 5 stacks of hemoplague. Darius's ult also costs no mana while the enemies are stunned. 10 minute cooldown. Resets if Darius gets a pentakill.