Umbreon is one of the seven evolutions for Eevee that was new in the Johto region. Umbreon benefits from having a high defense and special defense, as opposed to the other evolutions an Eevee could undergo. Its name comes from the words "Umbra", the darkest part of a shadow, and "Eon", a long period of time (Referring to evolution).
Evolving eevee into Umbreon is broken in Pixelmon 2.5.1.
To evolve an Eevee into Umbreon you need to level up during the night with at least a happiness of 220. The speculation that it can be evolved in dark places, such as caves or underground, is false.
To make an Eevee happy you have different options: