Umbreon is the first Dark-type Pokémon in National Pokédex order. It is numbered #197, making it the earliest Dark-type entry after the original Kanto Pokédex, which contains no Dark-type Pokémon.
Dark was introduced as a type in Generation II, alongside Pokémon Gold and Silver. Umbreon was one of the new Dark-type species added in that generation, and its National Pokédex number places it immediately before the Dark/Flying Pokémon Murkrow at #198. Other early Dark-types appear later, including Sneasel at #215 and Houndour at #228.
Umbreon evolves from Eevee when Eevee has high friendship and levels up at night, provided it does not evolve into another form. Its dark rings glow during battle, and the Pokédex describes toxins secreted from its sweat glands as part of its defensive biology.
The wording “first Dark-type” refers to National Pokédex order, not release order within a particular game or the first Dark-type encountered by a player. Umbreon is also the first Dark-type by number, even though other Generation II Dark Pokémon may appear earlier in a player’s personal adventure.