Yagg is one of the trials that travelers of Skornheim must pass. Skornheim is a deadly land just beyond Asgard, which Odin uses as a testing ground. When Loki and Thor were confronted by Yagg, Thor knocked him out (or killed him) by striking him in the head with a rock (Journey Into Mystery 116).
Yagg is a giant gladiator, who has electrodes for fists. He is able to discharge powerful surges of electricity at his enemies. When he brings the two electrodes together, the charge is strong enough to kill even a god of Asgard.