Vigrid is the final area of the game, reachable by traveling east from Asgard.
The good and evil gods are battling it out when you arrive. Both have ridiculously high hit point totals, so attempting to personally help with the attack is not recommended. In fact, the evil gods will one-shot you if you step in a space adjacent to them. Don't do that. Teleporting (randomly) is risky, but worth it if you haven't yet burned your potion of second life.
Your goal is to deliver all your remaining artifacts (Mimming, Mjollnir, Gungnir, and Tyr's arm) as quickly as possible (generally within 80 turns). To deliver each item simply stand next to the appropriate friendly god.
After everything is delivered, simply wait for the battle to end.
Generally, completion of 4 out of 6 quests is enough to win. However the actual outcome hinges on the (unclear) battle mechanics of the gods, so your mileage may vary.