Gellir is a Wight and former prince of the Danes.

Magnus Chase and the Gods of Asgard

The Hammer of Thor

When Magnus Chase 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 so he calls on his undead bodyguards to do so. The group distract them by turning the massacre into a thing - an early predecessor of parliament. After a brief delay, his fellow Wights attack Magnus and his friends who defeat them. Gellir himself is finished off by Hearthstone.