Sableye

Sableye is a Dark-Ghost Type Pokémon. It does not evolve. Its special abilities are Keen Eye and Stall. It hides in the darkness of caves. Its diet of gems has transformed its eyes into gemstones.

Pokémon Mystery Dungeon: Explorers of Time & Explorers of Darkness
In this game the Sableye are the servants of Dusknoir, and are constantly spying on and trying to capture the player pokemon in the world of the future.

Trivia

 * Sableye is the first of the two pokemon that cannot be hit super effectively, Spiritomb being the other.
 * Because of this, if a cheat devise is used to give Sableye the Wonder Guard ability, it will become invincible.