Mortal Shell has 4 melee weapons and one firearm. Read how to get a Hammer and Chisel.
All melee weapons (except for the Hallowed Sword, which you get in the prologue) are unlocked the same way: you need to find the book, interact with it and defeat Hadern, the character you fought at the end of the prologue. In doing so, he will use the weapon that you have to unlock.
To get the hammer, get to the Eternal Narthex. To do this, move to the right after leaving the tower, along the water forward, slightly to the left through the bandits’ camp, until you see the river along the cliff. There will also be a cave with a big toad. And there is the path among the monoliths to the left. You go there.