All eight of Eevee’s evolutions are included in the Pokédex in Pokémon Legends: Arceus—and Sylveon is the newest of them all.
To get all of the Eeveelutions, you’ll first have to find eight Eevees, nine if you want to keep a living Pokédex with the Pokémon pastures. Players can encounter a Sylveon without having to evolve it, but that method is used later in the game. The best and easiest method to get a Sylveon is to evolve an Eevee.
To evolve Eevee into Sylveon, players must max out their friendship level. During the course of your time with the Eevee you select to evolve into Sylveon, you can check how far your friendship is going with the Friendship Checker, which can be found in the game standing by the pastures.
In addition to having a maxed-out friendship level, Eevee must have a Fairy-type move. At level 18, Eevee can learn the move Baby-Doll Eyes, which would suffice for the evolution. If you missed that move while it was leveling up, you can head to the move shop and buy it.
Sylveon can also be found in the space-time distortions, which are described as dangerous places where powerful Pokémon appear. Sylveon appears in the space-time distortions that pop up in Obsidian Fieldlands or Coronet Highlands.