The ability to fuse with something or someone together to create a new, and generally more powerful, being.
Some users can also fuse together other living being, outside of themselves, as well as objects, powers, emotions, concepts, etc., turning them into single unit.
- Most duelists in Yu-Gi-Oh! (Via "Polymerization").
- Some Dragon Ball characters.
- Most Steven Universe characters.
- Soul of Cinder (Dark Souls)