Bior was the drighten, or leader, of a tribe of humans. He and his warriors rescued the Second Doctor, Jamie McCrimmon and Zoe Heriot from a rival tribe, led by Vignor. When Jamie was wounded, Bior had his men take Jamie to the tribe's medicine man, Hefn.
Bior and his men, and Jamie after he was cured, took part in a ritual every night that turned the men into bears. In this way they were able to protect their tribe. The Doctor didn't approve of this and tried to convince Bior that he shouldn't tamper with nature.
After Vignor's tribe raided Bior's village for food, Bior's son Einar was killed. In anger, Bior and his men, including Jamie, attempted to transform into bears to destroy Vignor's village. When Hefn refused to perform the ritual, Bior threatened to kill his own wife, Aella, and Zoe. Hefn performed the ritual, and after the men destroyed much of Vignor's village, the other men returned to human form, but Bior was stuck in bear form. (PROSE: That Which Went Away)