Eye of Ender

An eye of ender is a craftable item that can be thrown in the Overworld to show the direction of the nearest stronghold, which may break the eye. Up to 12 eyes of ender are required to activate a stronghold's End portal, which leads to the End dimension.

To locate strongholds (and the End portals they house):

Note that the eyes may point to an incorrect location if the target chunks were generated with a different biomes map in an older version or through different generation settings.[1]