The power to gain strength from fusion. Technique of Fusionism. Variation of Affinity.
User becomes stronger, faster, more durable, etc, from being fused to or infusing any object, power, matter or entity, possibly unlocking abilities related to the affinity and enhancing their existing powers. Some users may be able to draw sustenance from the things that they are infused with.
Caleb (Buffy the Vampire Slayer) merges with the First Evil...
...enhancing his strength to the point of sending Buffy flying across a room with one punch.
The Potara (Dragon Ball series) can not only fuse any two beings together, but the resulting fusion will have the full, maximum potential of the two multiplied tenfold, resulting in a being of incomparable power.
Piccolo (Dragon Ball) using his people's forbidden fusing technique to fuse with another Kami to gain a significant power boost.
C-Moon (JoJo's Bizarre Adventure) is a fusion of the stand Whitesnake and Green Baby, granting it has the power to control gravity.
Sora (Kingdom Hearts) can use his Drive Forms to temporarily fuse with his allies, granting boosts to his physical combat and/or magic skills.
In the Dream World, Mario (Mario & Luigi: Dream Team) fuses with Dreamy Luigi, enhancing his stats and giving him new, more powerful attacks.
Garnet (Steven Universe) is a fusion between two gems making her stronger.
As Shepherds, Sorey and Rose (Tales of Zestiria) can fuse with the seraphim to achieve powerful elemental forms.