It allowed the holder to have power over others and create portals between realms. Kano managed to find the crystal and had intended to use it for world domination, but after a vicious battle, Jax took the crystal from him and used it to teleport Kano and himself back to Earthrealm.
The power of this crystal may be how the Outerworld Investigation Agency was able to create the technology used for their inter-realm travel.
Legend of the Eye of Chitian
Legend has it, that the Eye of Chitian is the crystallized eye of a mythical sorcerer named Chitian, an Outworlder with the power to create interdimensional doorways to other worlds, though that has not been entirely proven otherwise.