King Sombra (or simply known as Sombra) is one of the secondary antagonists of My Little Pony: Friendship is Magic. He appears as the main antagonist in Season 3's premiere "The Crystal Empire" and in Season 9's premiere "The Beginning of the End". In the IDW MLP comics, he appears as the secondary antagonist in the 13th story arc Siege Of The Crystal Empire; the real main antagonist being his "mother" Rabia.
He is a tyrannical unicorn and the former ruler of the Crystal Empire who is bent on reconquering the empire and later taking over all of Equestria. In the IDW comics, it is stated that Sombra is actually an Umbrum created to destroy the Crystal Heart and release the Umbrum army trapped underneath the Crystal Empire; although this is considered to be non-canonical to the series.
And he is voiced by David Kaye in the English dub, and his Japanese voice is unknown.