You need to defeat the 3 Daughters of Lerion, and then use the swords received from them to open the cache to get this unique helmet of the God.
The Daughters of Lerion are powerful witch warriors. You will have to thoroughly upgrade the character to defeat them. For example, for the last one, the 300 power level is recommended. However, on easy difficulty it can be defeated even at level 150. At any time, you can lower it in the game settings. Go to the gameplay tab.
Where to Look for the Daughters of Lerion
The first is located in Grantebridgescire. The other two are in East Anglia. Their location is shown in the screenshots below.
Once you have collected all three swords, head to the cave marked in the screenshot below (in East Anglia):
At the very end of the cave there will be a statue that looks like Lerion’s daughter. There are 3 holes on her back. Insert three swords into them and then use the ladder to get to the equipment chest. Inside, there is Tor’s helmet.