The Medieval Sword is part of the Colossal Castle blueprint and block-set. You can buy either the Colossal Castle bundle, which gives you the recipes, or the Medieval pack, which gives you a sample of each block, but not the recipes.
You can create the Medieval Sword in your Crafting Menu (default key "Q") or at Crafting Tables (which still exist in older worlds) after the recipe has been unlocked upon buying it.
To craft a Medieval Sword, you'll need
- 2 iron rods (crafted in the processor)
- 2 iron bars (crafted in the processor)
- 1 warmworm tooth
- 1 unit of water
- 1 unit of lava (which means that you can craft the sword only after unlocking the lumite mining cell which allows you to pull lava).
To use any sword you must first equip it with a right click from the inventory or a quickslot until you can ready it for attack with TAB (default key).
Whenever you hit a creature with a sword, the weapon will lose a bit of durability until it will fall apart completely. The durability is indicated by a bar over the icon of the sword - in your equipment slot next to your character's portrait as well as on the main screen in the weapon slot. You might want to be prepared and take a new sword with you if your current one already hasn't much durability left!