Gellir is a Wight and former prince of the Danes.
When Magnus Chase and and his friends head to Provincetown to try and find Mjolnir, they enter his burrow. The group frees him from his tomb and he tries to kill them as thanks and calls on his undead body guards to do so. They distract him by turning his massacre into a thing- an early predecessor of parliament. After a brief delay, his men attack Magnus and his friends who defeat them. Gellir himself if finished off by Hearthstone.