"Weapon Customization
Weapons can be upgraded and customized on a per-voxel basis. For instance, iron swords can be upgraded with iron cubes which can be freely placed on the weapon. This increases damage and changes the look of weapons.
Spirit cubes are rare magic cubes that can be obtained by defeating bosses. They're more powerful than normal cubes and have additional effects when placed on a weapon:
-Fire Spirit deals additional fire damage.
-Wind Spirit increases attack and movement speed of the player.
-Ice Spirit slows attack and movement speed of the target.
-Unholy Spirit deals damage to the target and heals the player."
http://wiki.cubeworldforum.org/index.php?title=Customization#Weapon_Customization