Eldritch eyes are magical components necessary for teleportation within the world of TDDC. The eyes of any sufficiently powerful abomination will do, such as that of a goo person or the central eye of a Beholder (the eyestalk eyes are too small to contain enough energy, unless it's a magically altered beholder like the Beholdroid).
When a caster teleports, the vast energies of the astral plane depower the eldritch eye used, as well as any other eldritch eyes the caster has on their person. However, casters have found a way to link two eyes in a "round-trip ritual", in which one eye is left in a small shrine at the site from which the teleporter departed and another eye is used to perform the teleportation. When the caster wishes to return, they can use the eye in the shrine to perform a second teleportation back to the shrine. If the shrine is disrupted, the second teleportation cannot occur, although this does not affect the first teleportation if it has already occured.