North of Lirath there is a castle where a great evil lies waiting. Seek out Severan's help, and destroy the evil. The Ring of Power is your reward.
The quest:
From the Giant Oak tree north of Lirath, 4n, nw, 2n, up, 8n, pray to severan (skip praying if you're evil aligned), 2s, e, 2up, push brick, inspect sword (a spectre will attack), wall, 2 down, w, n, move tapestry, down, 3w, n, down, 3n, 4e, open coffin, get sword from coffin (You should now have Lightbringer.), 5w, open coffin (Gunthar autos if you're good; if you're evil he'll trade the sword for the Ring). And you are rewarded with a Ring of Power.
Tips:
It may or may not help to equip the Crystal Sword Lightbringer. And I have no clue if the Talisman of Severan helps in any way.
Rub the Ring of Power. It temporarily increases your guild's main stat by 1, and your guild's second main stat by 2. The ring cannot be used after any other stat boosts.
