Ninja Assassins are a powerful comp that uses the 6 assassin bonus and the extra damage from Ninjas (who are all individually strong units), to quickly destroy the backline. This comp does require level 7 and a Ghostblade to make work, but once you get there you will be very strong.
When to make
If you get an early spatula start and items that are good for assassins, then you can start to move towards this comp. It relies on Youmuu's Ghostblade to get you to 6 assassins at level 7.