The "Ultrakill" doesn't exist in League of legends.
There are titles for killing multiple enemies within a short time:
Doublekill - Kill 2 enemies
Triplekill - Kill 3 enemies
Quadrakill - Kill 4 enemies
Pentakill - Kill 5 enemies
Hexakill - Kill 6 enemies
There also was the "Legendary Kill" which required you to kill 6 enemies in a 5v5 game, however this has been removed since it was almost impossible.
Apart from the Double to Hexakill streaks, there are also titles for killing sprees that require you to kill a certain amount of enemies without dying.
Killing Spree - Kill 3 enemies without dying
Rampage - Kill 4 enemies without dying
Unstoppable - Kill 5 enemies without dying
Dominating - Kill 6 enemies without dying
Godlike - Kill 7 enemies without dying
Legendary - Kill 8 + enemies without dying
Shutdown - Kill an enemy on a killing spree
The killstreaks above will not be interrupted by being executed (a monster or turret kills you without any help of a player)