Here is the God Of War Ragnarök Rebel Leader trophy guide where we will help you return the Hammer of the Rebellion.
The Hammer of the Rebellion is the item you are going for during the favour Spirit of Rebellion which can be accessed after you have forged the Draupnir Spear and have ridden the train down with Freya, where, opposite the Mystic Gateway will be Durlinn asking to have a word.
If you ignore Durlinn here then you can go to the Tavern in the South East of Nidavellir (where you will then hand the Hammer over to complete the favour) and initiate the favour there instead.
Once you have spoken to Durlinn, kick the chain down and take the boat over to the East of Bounty Bay to Dragon Beach where you can then mantle up to the left, throw the spear into the wind section coming out of the strut then mantle up again.
Jump across the gap then interact with the green plate on the left and draw the symbol as prompted.
At the end of the bridge, have Freya interact with the Hammer once the Sigil is revealed and loot the chest.
Take the boat back down to main Nidavellir Boat Dock in the South East of Bounty Bay and hand the hammer to Durlinn to unlock your trophy.