Tirek

Tirek also known as Lord 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.

Development and Design
Tirek shares his name and certain aspects of his design with a Generation 1 villain from the animated television special Rescue at Midnight Castle. He is a centaur with dark reddish skin, white hair, black fur, cloven hooves, and is somewhat bovine in appearance.

Over the course of Tirek's appearances in the finale, his appearance gradually changes as he absorbs more magic. He intially appears very frail, with muted colors and a hunched posture that leaves him roughly at the height of a normal pony. After absorbing the magic of many unicorns, he gains a more muscular physique, brighter colors, and a pair of curved horns. His later transformations exaggerate his form even further, becoming larger, brawnier, and with increasingly long and elaborate horns.

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.

Powers
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.

In his first appearance, Tirek is shown to be able to consume the magic of ponies, removing their cutie mark in the process - he seems to literally suck this magic into his mouth, like a more kid-friendly version of "soul stealing". A pony drained this way is completely stripped of all magic. Tirek adds the stolen magic to his own, increasing his size and power. The more magic he drains, the more powerful he becomes. After devouring enough unicorn magic, he becomes able to strip pegasi and earth ponies of their innate magic as well. At very high levels of power, he becomes able to strip even an alicorn or a draconequus of all magic. If Tirek drains the magic of every single pony in Equestria, he effectively becomes its God, able to single-handedly control all the aspects of the land that ponies normally regulate collectively.

IDW comics
My Little Pony: FIENDship is Magic Issue #2 explores Tirek's "secret origins" at a young age, Tirek, a prince of his homeland, was instructed in the ways of magic by a centaur hermit named Sendak the Elder and became interested in pony magic through him. Tirek's ambitions greatly concerned his parents King Vorak and Queen Haydon.

When Tirek attempts to absorb the magic of a unicorn that Sendak captured, it backfires, resulting in an explosion that catches the whole kingdom's attention. King Vorak sends Sendak to the mines for his crimes, punishes Tirek, and returns the unicorn to Equestria. While confined to his castle quarters, Tirek plots to overthrow his father and rule over Equestria.

Trivia

 * Tirek will become Little Bear's enemy in Little Bear's Adventures of My Little Pony: Friendship is Magic - Twilight's Kingdom.
 * Tirek returns and guest stars in Brian and the Eeveelution Family's Adventures of The Lord of the Rings: The Fellowship of the Ring, Brian and the Eeveelution Family's Adventures of The Lord of the Rings: The Two Towers, Brian and the Eeveelution Family's Adventures of The Lord of the Rings: The Return of the King, Thomas and Twilight Sparkle's Adventures of Rambo III, Thomas and Twilight Sparkle's Adventures of Star Trek: Beyond, Thomas and Twilight Sparkle's Adventures of Blade Runner 2049, Thomas and Twilight Sparkle's Adventures of Dragonheart, Thomas and Twilight Sparkle's Adventures of Dragonheart: A New Beginning, and Thomas and Twilight Sparkle show How to Train Your Dragon: The Hidden World.
 * In a running gag in our heroes call Tirek a "Monkey Horse". But however, only a few heroes do call him a Centaur.
 * 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.
 * In his fully-powered state, Tirek is easily the most powerful antagonist (and maybe even character) in G4 (much as he was in G1). After consuming enough magic, he surpasses even Discord in power (a character who is constantly portrayed as near-omnipotent). However, despite the raw power he possesses, he is not as skilled with it as the draconequus is. Tirek mostly uses the magic of others to fuel his own familiar offensive spells, rather than reshaping reality at will like Discord does.
 * He is considered as one of the most evil villains in Friendship Is Magic, along with Queen Chrysalis, Cozy Glow and King Sombra. However, while those two villains were never presented with any attempts at befriending them, Tirek is shown to have consciously flat-out rejected the very concept of friendship with the ponies of Equestria, his brother Scorpan, and Discord, believing it to be weakness.
 * When Tirek steals Equestrian magic, he has the Xenomorph Queen's hissing sound.
 * Tirek also reveals to Tai Kamiya he made Tai's sister Kari Kamiya sick from the flu and she almost died.
 * Tirek will become Thomas' enemy in Thomas' Adventures of My Little Pony Friendship is Magic - Twilight's Kingdom (Full Movie)
 * Tirek became Aku's General in The Night to Remember.
 * He will appear in Team Robot in Pokémon the Movie: Hoopa and the Clash of Ages.
 * He make his return planning to capture all Legendary Pokémon and have revenge in War of the Apocalypse.
 * Tirek will be the third villain to be the main antagonist of two two-part episodes, other being Starlight Glimmer and Queen Chrysalis.
 * He is voiced by Mark Acheson, who also voiced Unicron in Armada and Energon of the Transformers Unicron Trilogy.
 * In Ryan F-Freeman's Adventures, Tirek is rivals with Ryan F-Freeman and one of his enemies who would never reform.
 * He will appear in Jewel Sparkles' Adventures as a villain.
 * Tirek become the enemy of Connor Lacey and his friends in The Irelanders' Adventures of My Little Pony: Friendship is Magic - Twilight's Kingdom.
 * He was Scand into the Omitrix to unlock Man-Te-Corn.
 * In Noah's Adventures Series Tirek was Never Evil. He Joins Noah on his Adventures.