The Essence of Void is one of three mob drops used to craft the Ender Pearl (changed recipe), the others being the Endstone Dust and Ender Pearl.
The Essence of void is dropped by EnderLords, and contains the remnants of their Endergy. This is used to track down portals to the End. One of these is required for each EnderPearl, meaning a total of 12 EnderLords need to be killed, along with 84 EnderBarons. Once this task is complete, you can face The End.
IMPORTANT: This item has not been implemented into the game yet.