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.