Souleon is the Ghost-Type evolution of Eevee. It evolves when you take it to Death Cavern(Pokemon Lunar and Eclipse only). It will run off, dissapearing into the shadows. You'll hear a yelp, and you feel a presence. When you follow Eevee's path, you see a white shape. It seems as though your Eevee died, but it really evolved due to being exposed to Death Cavern's high deposits of Ghost Stones.