Weapon fusion is a critical mechanic for increasing your weapons' power and customizing their abilities. At the Blacksmith, you can fuse two weapons together: a base weapon and a material weapon. The material weapon is consumed, and its experience points are added to the base weapon, increasing its level and base attack power. More importantly, seals from the material weapon can be transferred to the base weapon.
Seals provide various buffs, such as increased attack speed, stronger critical hits, or bonus damage against specific enemy types. Aim to fuse weapons with matching seals (e.g., two square seals) to create a 'perfect' seal, which provides a significant stat boost. Prioritize seals that complement your character's playstyle and the weapon's inherent strengths. For example, for Link's two-handed weapons, focus on strong attack damage and weak-point gauge damage seals.