Hefn was the medicine man of the tribe led by Bior.
He saved Jamie McCrimmon's life after Jamie was wounded in an attack by Vignor's men. Every night, Hefn performed a ritual that turned the men of the village, including Jamie when he was cured, into bears to protect the village.
Hefn, convinced by the Second Doctor that the men should not transform into bears to enact vengeance on Vignor's tribe, refused to perform the ritual after Bior's son Einar was killed. Bior threatened to kill his own wife, Aella, and Zoe Heriot if Hefn refused. Even though Aella tried to convince him to refuse, he gave in and performed the ritual.
After Bior became stuck in bear form, Hefn and Aella worked with Vignor's tribe to make peace. (PROSE: That Which Went Away)