Eye of Ender is an item added by vanilla Minecraft. Eye of ender is a vanilla item that is used to find strongholds. This item briefly drifts in the direction of a stronghold before dropping on the ground. It occasionally disappears after use and cannot be picked up. In Hexxit, it's used in several recipes. You can find Eyes of Ender naturally in Better Dungeons chests.
Other than for finding strongholds, it can be used to many crafting recipes