Fusion

Fusion is the system in which two or more demons are combined together to create a new demon. A demon created via this system will inherit certain traits and abilities from the demons that were used to create it. The exact nature of this inheritance varies by the types of demons used in the fusion, by the type of demon being created by the fusion, by the phase of the moon, and by various other factors that vary by game.

Some demons cannot be recruited or captured normally, and must instead be created via fusion. Some of these demons can only be fused once certain prerequisites have been met; for example, some bosses can only be fused once they have been defeated.

In several games, demons can also be fused into weapons or other equipment, giving that gear new abilities based on the abilities of the demon used in the fusion.

Special cases
In the Persona series, Personae can be fused in the Velvet Room in the same manner that demons are fused in most other games.

Fusion does not exist in the Digital Devil Saga: Avatar Tuner series. Instead, each character is limited to using one Atma Avatar, although they can gain new abilities for their Avatar by learning new Mantras.