Tirek

Tirek is the main antagonist of the Season Four finale episodes Twilight's Kingdom Parts 1 and 2. He is an escapee from Tartarus a dark land that serves as a prison for many horrific entities, and possesses the power to drain ponies of their magic. This Tirek is significantly different from his older counterpart - he has no army nor access to the Rainbow of Darkness, and instead relies on his own magical powers - but he shares the old Tirek's status as the most powerful and evil villain of his generation so far.

In Rescue at Midnight Castle
He resides in the forbidden Midnight Castle alongside his minions and his servant/slave Scorpan, who he orders to kidnap the ponies of Dream Valley to execute his evil scheme.

Tirek wields the Rainbow of Darkness, a powerful satchel containing evil magic which corrupts everything it touches. It can even turn the ponies he captured into evil dragons which were used to pull his "Chariot of Darkness" and execute the event called the "Night that Never Ends", on which he will use the Rainbow of Darkness to corrupt Dream Valley and plunge it into chaos. When Tirek takes off in his Chariot of Darkness, Megan grabs the satchel containing the rainbow of darkness, but can't hold on to it. As he was about to release the Rainbow of Darkness, Megan opens the locket to reveal the Rainbow of Light. But it's only a small piece that is soon enveloped in darkness. But the Rainbow of Light starts fighting back, and eventually overpowers the Rainbow of Darkness. Tirek was destroyed by the Rainbow of Light, freeing Scorpan from his spell and turning him back into his human form.

In Twilight's Kingdom
Tirek and his brother Scorpan tried to take over Equestria; but when Scorpan became friends with Starswirl the Bearded, he came to appreciate Equestria and tried to turn Tirek away from his quest. Tirek refused, so Scorpan warned Princess Celestia and Princess Luna, who were able to imprison Tirek in Tartarus.

When Cerberus left his post at the gates of Tartarus (in episode "It's About Time; for unknown reasons) Tirek was able to escape. Sometime after he began absorbing a unicorn's magic. Celestia received a vision of this and sent Discord after Tirek. However, Tirek was able to manipulate Discord into betraying his friendship with Fluttershy and joining his side. Tirek began eating energy from pegasi and earth ponies to take away their flight and strength. Finally he went after Celestia, Luna, and Cadance, but finding their magic to be gone he imprisoned them in Tartarus. Discord thought Tirek didn't value him as a partner so Tirek gave him his medallion, saying that it was his only keepsake from someone close to him. Then Tirek saw a stained glass window of Princess Twilight Sparkle, and Discord explained to him who this new princess was. He and Discord captured Rainbow Dash, Fluttershy, Applejack, Pinkie Pie, Rarity, and Spike, and Tirek ate their power before doing the same to Discord, saying that his medallion actually came from Scorpan, and that Discord is as worthless to Tirek as Scorpan was.

Growing to tremendous size from the magic he absorbed, Tirek attacked Twilight, and when he destroyed her library, she became enraged and fought him in an epic battle. The two were evenly matched, though Twilight seemed to have a slight advantage, so Tirek revealed the other Mane 5, Spike, and Discord, imprisoned in orbs. Tirek told Twilight to give him her magic and that of the three Princesses, and she agreed, so Tirek released all except Discord. Twilight told Tirek to release him as well. He accepted and then ate Twilight's magic, increasing his size to massive proportions. To thank her for her kindness, Discord gave Twilight Scorpan's medallion which was the final key to the Tree of Harmony box. Unlocking it, the Mane 6 received Rainbow Power, which they used to blast Tirek with a rainbow not unlike the one produced by the Elements of Harmony, leaving him imprisoned in a cage in Tartarus with no power while restoring Equestria and the ponies to the way they were.

Trivia

 * Tirek was originally a G1 villain who was the main antagonist of Rescue at Midnight Castle.
 * While Tirek was likely meant to outshine previous villains such as Chrysalis and Sombra, many fans hate him for destroying Twilight's library and sending the princesses to Tartarus. Some have even speculated, out of hate for Tirek, that after his reimprisonment, Cerberus eats him as punishment for sending the princesses to Tartarus. This is the exact opposite scenario of Chrysalis and Sombra; When they died, most fans denied it out of love for them or desire to see them return (and the IDW comics, whose writers are bronies and pegasisters, had Chrysalis survive, while some continue to deny Sombra's demise even after it was confirmed). This, in turn, could likely be the reason why Meghan did not have Tirek end up like his G1 counterpart.
 * Tirek will become Thomas' enermy in Thomas' Adventures of My Little Pony Friendship is Magic - Twilight's Kingdom (Full Movie)
 * Tirek will get his revenge on Thomas and Twilight in Thomas and Twilight Sparkle Visits The Haunted Mansion, 'Thomas and Twilight Sparkle's Adventures of Disney's Villains' Revenge and Thomas and Twilight Sparkle in Fantasmic (Walt Disney World Version).
 * Tirek will guest star in Pooh and Brian Griffin's Adventures of Transformers: Age of Extinction and Brian Griffin vs. The Mummy: Tomb of the Dragon Warrior.
 * Tirek is also The Diesel Squad's arch rival.
 * It is revealed in Pooh and Brian Griffin's Adventures of Transformers: Age of Extinction, energy crystals give an even better power than magic does and in Brian Griffin vs. The Mummy: Tomb of the Dragon Warrior' when he is shot at by guns, explosives, and other modern weaponary he loses his power he has aqulataed on and shinks back to his weakest form.
 * Tirek is killed in Brian Griffin vs. The Mummy: Tomb of the Dragon Warrior but returns in Brian Griffin's Adventures Chronicles.
 * Tirek is friends with Hunter before he went to Tartarus and was killed by our heroes and then Megatrain.