Mortal Shell has 4 melee weapons and one firearm. Read how to get the Martyr’s blade.
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.
Martyr’s blade can be obtained at the beginning of the Abandoned Chamber, outside the Fallgrim Outskirts. The book is literally at the beginning of the location. It stands on a stand every time, next to the statue. To get to the Fallgrim Outskirts, go to the right of the tower (leaving it through the door), get to the ascent up, but follow a little to the left. At the fork with the sign, go left. On the left hand will be a camp of robbers. Kill them and look for the gorge. And then – go forward until you find the entrance to the Abandoned Chamber.
This weapon is a huge sword – slow, heavy, but inflicting incredible damage.