Fusion

Fusion (融合, Yūgō) is the ability for an entity to coalesce with one or more entities via artificial or natural means. Usually, the powers of the combined entities merge into one and comes with a significant boost in power. Control of the resulting fusion may be shared or taken by the entity with more power.

Overview
This ability allows the user to merge with other entities, sometimes against their will, in order to obtain a significant power enhancement.

Orochi has the ability to absorb the flesh and blood of other creatures in order to regenerate itself. In the presence of an extremely powerful target, it will attempt to absorb it's powers as well, as seen with Psykos.

Psykos may not only fuse with other entities similar to Orochi, the vast extents of her Psychokinesis allow her to combat the invasive touch of Orochi and successfully gain full control of the resulting fusion. This gave her a huge power boost during her battle with Tatsumaki that even enabled Psykos to give the esper hero trouble.

Drive Knight can fuse with other robotic entities in order to combine each other's cores and obtain maximum power output. This is known as Tactical Combinations, allowing more proficient use of his shape-changing box in order to gain promoted versions of his regular Tactical Transformations. It should be noted that it's functionality depends on the compatibility of Drive Knight's robotic partner. First used via merging with Genos to create the transformation: Promoted Rook against the fusion of Psykos and Orochi.

Genos was sufficiently compatible with Drive Knight to fuse together. First used via merging with Drive Knight to create the transformation: Promoted Rook against the fusion of Psykos and Orochi.

Baquma has the ability to inhaling monsters through his trunk while digesting them, Baquma could temporarily utilize said monster's abilities. Whenever he did so, he gained a third eye and an extra trunk for each new power he obtained. However, after fully digesting the monster and excreting it, he will lose the powers.

Black Sperm, a monster who consists of trillions of living cells, is able to merge his cells together to create a much more powerful version of himself. The strength of the resulting form depends on the amount of cells fused into it, and the fused form is able to absorb other non-fused cells. However, Black Sperm's fused forms are unable to separate unlike their components, not even to regenerate from mortal damage.