Sword fusion

Sword Fusion is a system in certain games in which a demon can be fused into a blade (or another weapon) to enhance its properties.

Devil Summoner: Soul Hackers
Sword Fusion can be performed at the Hotel Goumaden with Muramasa. This process sacrifices a demon to be fused into the Nameless Sword, changing its properties. In some cases, a special weapon is created when certain demons are fused into the blade.