Thor's Battle Plate
Daughter of Lerion - Walsham Crag - East Anglia
Defeat Regan, Daughter of Lerion in East Anglia
Daughter of Lerion - Grantebridgescire
Defeat Goneril, Daughter of Lerion in Grantebridgescire.
Finish the "Breaking The Order" quest line by delivering all the medallions to Hytham.
Cordelia - Daughter of Lerion - East Anglia
Defeat Cordelia, Daughter of Lerion in East Anglia.
Lerion Crypt - East Anglia
Go to the Gear marker in the south of East Anglia.
In the ruins, you can go down a Crypt, where you can insert the 3 daggers of the 3 Daughters of Lirion you defeated.
It will open up a Treasure Vault with Thor's Helmet as main price.