It creates throwing stars out of compressed water. When it spins them and throws them at high speed, these stars can split metal in two.
– Pokémon X Pokédex
Ash-Greninja started off as an anime-only form of Ash’s Greninja, when it sort of fuses with Ash. It’s not quite a mega evolution since it doesn’t require a mega stone, and only Ash’s Greninja can do this transformation because reasons. I thought it was fine as an anime-only thing….
…until the demo of Pokemon Sun/Moon made it a thing in the games as well. You can get a special greninja with the Battle Bond ability, which makes greninja transform to this form after fainting an opponent. It gets a mega-like stat boost (+110 bst), an boost to Water Shuriken’s base power. Since it’s akin to an event Pokemon, it cannot breed.