Weavile evolves from Sneasel when Sneasel levels up at night while holding a Razor Claw. This evolution method was introduced in Pokémon Diamond and Pearl, where Weavile became Sneasel’s new evolved form.
The Razor Claw is both a held battle item and an evolution item. For the evolution to occur in the traditional games, Sneasel must be holding it when it gains a level during the game’s nighttime period. Simply using the item or leveling up during the day will not work.
Players commonly confuse Weavile with Sneasler, another Sneasel evolution. Sneasler evolves from Hisuian Sneasel during the daytime, also using a Razor Claw. The regional form and time requirement are therefore important: regular Sneasel becomes Weavile at night, while Hisuian Sneasel becomes Sneasler during the day.
Weavile is a Dark- and Ice-type Pokémon, whereas Sneasler is Poison- and Fighting-type. In Pokémon Legends: Arceus, the evolution rules use the same day-and-night distinction but present the process somewhat differently through the menu.