N-Solarizer
From Pixelmon Wiki
This is an item that can be can be used on Necrozma to fuse with Solgaleo into Necrozma-Dusk Mane or to separate Dusk Mane Necrozma into its constituent Pokémon. When you have Necrozma and Solgaleo in your party, and you have them both out, you can fuse them by right-clicking into one of them with the N-Solarizer.