The Gunpowder is needed for you to enter the cave in the Viking era in Time Tangled Island. You will need the barrel of gunpowder from the Great Wall of China (6 o’clock, 1593 AD), which is where you’d be going for the Thor Amulet as well. Both are on the right side of the Wall. Bring the gunpowder to the entrance of the cave on the hill at the Viking island. It will blast the rocks away and you can then enter the cave.
How to Get the Gunpowder
Use the Time Device to go to the construction of the Great Wall of China (6 o’clock, 1593 AD), keep going on the right and you will find a barrel of gunpowder.