Eevee evolves into Umbreon when it levels up with high friendship at night. This evolution method was introduced in Pokémon Gold and Silver, where Umbreon debuted alongside Espeon as one of Eevee’s second-generation evolutions.
Friendship is a hidden relationship value that rises through activities such as travelling with a Pokémon, levelling it up, grooming it in some games, and using certain items. To obtain Umbreon in the traditional games, the player must build friendship and then level Eevee during the game’s nighttime period. Levelling it during the day instead produces Espeon, provided the other conditions are met.
Umbreon is a Dark-type Pokémon, while Eevee itself is a Normal-type Pokémon. The method can fail if Eevee knows a Fairy-type move in games where that affects evolution rules, or if the player uses an evolution stone. Other Eeveelutions use different triggers, including elemental stones, locations, and moves.