The Enderfested effect acts similarly to the Infested effect, but spawns Endermites instead of Silverfish.
Whenever an Enderfested entity is hurt or takes damage by any means, there is a 10% chance to spawn 1-2 Endermites flung in the direction the entity is facing. The potency level of the potion does not matter; it does not change anything.
You can make Potions of Enderfestation by adding an Ender Pearl to an Awkward Potion.