Thor’s Helmet is obtained from the Crypt of Lerion, which is only accessible once you’ve slain all Daughters of Lerion. The Crypt of Lerion is found in East Anglia, and the entrace to the crypt is unlocked by the daggers of the three daughters. The final Daughter of Lerion boss, Cordelia, is power level 340 so you may need to wait until you’re a higher level before you can unlock this trophy.

View Full-size

Cordelia is the strongest Daughter of Lerion.