Father Glowworm is a supporting but pivotal antagonist in the Avatar: Kyoshi novels, being the impetus of the events that lead to both Kuruk's demise and (alongside Jianzhu) Yun's descent into villainy.
Father Glowworm has the appearance of a glowing eyeball the size of a wagon wheel attached to pulsating veins. He also maintains a blood-thirsty aura around him.
An ancient spirit even older than the likes of Koh the Face Stealer, Father Glowworm had a thirst for human blood, and had the power to dig tunnels that created rifts between the mortal realm and the Spirit World, which he used to feast on human blood. His rifts also allowed dark spirits, angry at Avatar Yangchen's fondness of humans, to wreak havoc in the mortal world. Kuruk would find and defeat these dark spirits at the cost of his own spritual and physical well-being. One of the spirits directed Kuruk towards Father Glowworm, who was responsible for creating the rifts. When they met, Kuruk made the mistake of introducing himself by name, which Father Glowworm was able to use to put a life-draining curse on Kuruk for merely knowing his existence. The Avatar and the ancient spirit battled, and although Father Glowworm was stronger, Kuruk weakened Father Glowworm enough for him to retreat and remain inactive for years, ending the dark spirit threat for generations.
At some point prior to the events of the Avatar: Kyoshi novels, Father Glowworm was encountered by Jianzhu, one of Kuruk's old friends and mentors. Father Glowworm presumably put the same curse on Jianzhu as he did on Kuruk, and though it likely contributed to the circumstances surrounding Jianzhu's death, he still lived much longer than Kuruk.
Present Day and Demise
Desperate to find the true identity of the Avatar, Jianzhu brings the two candidates, Kyoshi and Yun, to Father Glowworm, who gets samples of their blood. After identifying Kyoshi as the true Avatar, Father Glowworm became enraged at her being the incarnation of Kuruk, his greatest enemy. Jianzhu saved Kyoshi but left Yun for dead as Father Glowworm dragged Yun back into the Spirit World.
Desperate to return to the human world, Yun feigned defeat and begged Father Glowworm to become his master. Though he saw through Yun's ruse, he was amused by Yun's request, but quickly put him down by belittling and dehumanizing him, reminding him of his inferiority to the ancient spirit. He told Yun that although he would not escape the Spirit World, he would promise him immortality. Yun and Father Glowworm battled one another, and despite Yun being a regular human, he managed to weaken Father Glowworm had been weakened and his form had grown much smaller. Father Glowworm offers a truce and, in exchange for bringing him human sacrifices, offers to share his power to create rifts between the Spirit World and the human realm. Yun seemingly accepts, but after getting close enough to the spirit, Yun traps the spirit. Father Glowworm angrily shouts that even Kuruk was not as dishonorable, but Yun did not care for the spirit's words as he consumed Father Glowworm's eye, killing the ancient spirit and gaining his powers as a result.