Rabia

Rabia is the leader of the Umbrum in the My Little Pony: Friendship is Magic comics, and the true final antagonist of the comic Siege of the Crystal Empire.

Background
Rabia and the Umbrum army attacked the Crystal Empire, terrorizing Crystal Ponies and burning their homes, until they were imprisoned in another realm.

Fiendship is Magic Issue 1
In Sombra's comic backstory, Rabia disguised the Umbrum, King Sombra, as a pony so he could go through the prison door and release them. When Sombra did not want to turn evil and ran, Rabia communicated with him through the crystal and he embraced his dark side.

Siege of the Crystal Empire
It is shown that after Sombra was banished, Radiant Hope found Rabia and the Umbrum, and when she entered their prison, they made her think they were good, and were simply misunderstood by ponies. This led her to resurrect Sombra so he could release the Umbrum. However, when Hope brought Cadance to the Umbrum prison to show her that they weren't evil, Cadance's light revealed their true selves. The Umbrum attacked Hope and Cadance, but they escaped.

Eventually Sombra released the Umbrum, and Rabia put Twilight and Cadance on trial, and demanded to know where the rebel ponies were. When Cadance told her they were on Canterine, Rabia had Sombra turn Shining Armor to stone anyway. Twilight convinced them to take them to the dais where the Crystal Heart was before turning them to stone so the Crystal Ponies could witness their defeat. Then the rebels attacked, the other Mane 5 among them, but Rabia defeated Rainbow Dash and other pegasi with a flyswatter and told Sombra to destroy the Crystal Heart. However, after hearing of how Hope chose a different destiny than the one the Crystal Heart showed her, and chose him over becoming a princess, Sombra switched sides and used the Crystal Heart to banish the Umbrum, including Rabia, back to their prison.

Trivia

 * The exchange between Rabia and Princess Cadance when she tells King Sombra to turn Shining Armor to stone is the same as when Grand Moff Tarkin destroys Alderaan and Princess Leia tells him the rebels are on Dantooine (which, in Equestria, is called Canterine).
 * It is unclear as to why Rabia and the other Umbrum were only banished by the Crystal Heart's power when it is fatal to Sombra (who is also an Umbrum).