The Pokémon series has had special evolution requirements for certain Pokémon since the beginning of Generation I. When a Pokémon from a prior generation comes into a newer game, like Pokémon Scarlet & Violet, generally, the requirement to evolve stays the same. Misdreavus, a Generation II Pokémon, has had a special evolution since Generation IV, and you can use this method in Paldea. Here's how to evolve Misdreavus into Mismagius.
How you can evolve Misdreavus into Mismagius in Pokémon Scarlet & Violet
To evolve Misdreavus into Mismagius, you'll need to use a Dusk Stone on it. This is the same item that Murkrow requires to evolve into Honchkrow. Find the Dusk stone in the Other Items sub-menu of your bag to use it. A candy icon on the top of the screen indicates it. Select it, select Use this item, then select Misdreavus.
Where to find a Dusk Stone in Pokémon Scarlet & Violet
If you caught Misdreavus early on in Pokémon Scarlet & Violet, you might have to wait some time to evolve it. Dusk stones won't really be available to you until much later. You can occasionally find them available for sale at the market in Port Marinada. Your other option is to travel to Montenevera and look behind Ryme's Gym. There is a Dusk stone inside a Red Pokébell there.
Wondering about Eeveelutions and how to get them? Check out How to evolve Eevee into Flareon in Pokémon Scarlet & Violet, How to evolve Eevee into Leafeon in Pokémon Scarlet & Violet, or How to evolve Eevee into Jolteon in Pokémon Scarlet & Violet here at Pro Game Guides.