Mortal Shell has four weapons to find, three via a boss battle.
Let's start by eliminating the basics. The Sacred Blade is in the tutorial, so everyone will have access to it from the start. Marty's Blade, Hammer and Chisel, and Smoldering Mace are all buried a little deeper in Mortal Shell, but they don't take long to grasp if you know where to go.
In the main center of Mortal Shell is the Fallgrim Tower, and at the top is a wall where your weapons are stored. Any weapon you don't own can be located via some sort of echo after interacting with its empty cloak. Or, you can just do a lightning attack at the entrances to the three temples of Mortal Shell. All three have a book at the start where the Sester and Workbench rest, and interacting with them will trigger a boss fight against an enemy wielding the weapon you're trying to win. From here you can find the two items needed to unlock weapon abilities deeper into the temple. None are hard to find: in fact, they're all on the critical path, so you'll only miss them if you ignore the loot.
The weapons are:
- Hallowed Blade (Tutorial)
- Standard weapon with medium speed and damage. Can deploy a spike from the front and rear for massive damage when leveling.
- Martyr's Blade
- Giant Greatsword with heavy damage, but slowly swings and eats stamina. Abilities imbue it with frost or provide a wide area of attack that freezes enemies caught in the blast.
- Hammer and Chisel (Seat of Infinity)
- The dual-use option: attacks quickly and consumes little stamina, but doesn't hit very hard per attack. Abilities deal heavy direct damage or a spinning attack area for decent damage to enemies within range.
- Smoldering Mace (Ash Sanctuary)
- A mass that acts like a pit like a gun; slightly faster than Martyr's Blade and nibbles less stamina, but deals less damage as a result. The abilities are the same as the Martyr's Blade, but swap frost for fire.
A few final notes: You can't switch weapons willy-nilly Deadly Shell until you find an item that is outside of their respective temples in a chest accessible only when Fallgrim is covered in heavy fog. (killing the temple boss will trigger it, and after your first boss has been killed you can manually trigger it at the Fallgrim temple). Additionally, you can trade at the start of a temple near the book, but you will only trade what you have with what there is. However, you can switch between any weapon unlocked in Fallgrim Tower. A little stuffy to be sure, so don't forget to loot these weapon items when you leave a temple after killing the boss.