The Ring of Zork is a magic sapphire ring owned by Syovar. Hismagic powers flow from it and he is never seen without it, and actually when he is seen, it means that it is not him, but an impostor, like Krill.
However he parts with it in order to offer it to the heroes Bivotar and Juranda. Doing so they are transported back in their homeworld. It is thanks to this Ring that they can return to the Kingdom of Zork.
Some time after their first adventure, the kids received a message from Syovar in their dreams. Bill checked the ring and it saw it flashing and decided it was not coincidence, and wore it to be transported to Zork.
After their second adventure, Bill was examining the ring while sitting at the Lookout Point; then a lizard startled him and he dropped the Ring, which was about to fall off the edge. They tried to catch it but they stumbled and fell from the dedge too; but the power of the Ring protected them and they were transported to the Castle of Zork. Soon after the Ring was also materialized in the air and fell on the floor. After their adventure in the Cavern of Doom, Syovar returns it to Juranda.
Although Ellron says that the Ring is Syovar's source of power, and never leaves it, Syovar still performs powerful feats of magic even after he has passed it to the kids.