In Norse mythology, Ragnarök is the Final World Battle, a series of events that results in the deaths of the Norse Gods and the end of the world. After which the world ends, it is to be reborn again. The god Odin prepared for the Battle by having Valkyries scout for the souls of worthy warriers who died in battle. They would take them back to Valhalla to train them for the final battle between Good and Evil.
In 2003, three Valhalla Warriors were transported to the city of San Francisco after chasing the Charmed Ones through a portal. Confused by all the evil in the city, the warriors believed that the final battle has begun and started killing innocent mortals. When the Valkyries arrived to collect them, they went into battle until they were able to convince the warriors it was not the final battle by opening a portal back to Valhalla. The warriors then returned to their island. ("Valhalley of the Dolls")