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Lucumo: How to open that one door? Was randomly exploring the world and stumbled over it. Beat that monster thing in the North but can't open the door a bit back.

Also, is the game that easy or am I overleveled (was level 15 when I beat that monster) for this? It's not like I killed monsters on the side, have only followed the quests so far (until I saw the path down there while escorting the head).

PS: Will I ever be able to swim? In the temple there are some ladders and ways but they are inaccessible because of the water.
It's part of one of Wyrm Hunt quests you can get from Ser Maximillian later in the game. Can't be explored fully until then.