The Eye of Rhynn is literally the right eye of the Lost God Rhynn. It was inserted into Prince Corum's empty orbit after his eye was removed, and it allows him to see into a dark netherworld, possibly Limbo or one of the infinite hells, and with the help of the Hand of Kwll, to reach into the netherworld and summon it's undead denizens to slay Corum's enemies.
In appearance, it is a large orb, larger than a human eye, and is covered with jewels, giving it an appearance much like the eye of an insect, which is a feature common to Gods and Angels in the Multiverse.