Depiction in the seriesEditCelestia and Luna explain to Twilight Sparkle that Tirek and his brother Scorpan came to Equestria from "a distant land" to steal Equestrian magic. However, Scorpan grew to appreciate the ways of Equestria and even befriended a young unicorn wizard. Scorpan pleaded with his brother to abandon their plans, but Tirek refused to relent. To save Equestria, Scorpan informed the princesses of Tirek's intentions. Scorpan returned to his homeland while Tirek was imprisoned in Tartarus for his crimes.
Celestia states that Tirek likely escaped during the events of It's About Time, when Cerberus left his post at Tartarus' gates, and has only recently regained enough strength to use his dark powers after languishing in Tartarus for centuries.
While continuing to devour unicorn magic, Tirek is captured by Discord, who is there on behalf of his friendship to the ponies. Likening Discord's actions to those of his "weak-minded brother," Tirek appeals to Discord's former nature as a master of chaos, convincing him that friendship is "a new form of imprisonment" and that betraying the ponies will grant him freedom to do as he pleases. With Discord's aid, Tirek is able to continue absorbing unicorn magic, gaining a stronger form and increasing in power. He soon becomes strong enough to begin absorbing magic from Pegasi and Earth ponies as well.
In Twilight's Kingdom - Part 2, Tirek continues to grow in strength and size. He becomes powerful enough to steal Alicorn magic and makes his way to Canterlot Castle. Anticipating his arrival, Celestia, Luna, and Cadance transfer all of their magic into Twilight and send her into hiding, believing Tirek is unaware of her royal status. When Tirek arrives and finds the princesses devoid of their magic, he banishes them to Tartarus as revenge for his own imprisonment.
When Discord has doubts about their alliance, Tirek gives him the medallion around his neck as a token of his gratitude and loyalty, claiming that it originally belonged to "someone very close" to him. He soon discovers Twilight via her depiction in a stained glass window and questions the existence of a fourth princess; Discord confirms this and directs Tirek to Twilight's home in Ponyville.
When the two arrive, they capture Twilight's friends and Tirek absorbs their magic, growing even more powerful. Discord is thrilled by the prospect of ruling Equestria alongside Tirek, but Tirek suddenly turns on him and says he is no longer of any use, proceeding to absorb his magic as well. When Discord reminds Tirek of the medallion, Tirek reveals it was given to him by his treacherous brother Scorpan and considers it "as worthless as he is."
Tirek soon finds Twilight and pursues her relentlessly, destroying the Golden Oak Library in the process. Enraged, Twilight fights Tirek in an intense battle of magic. Tirek fights back, but their evenly matched might leads them to a stalemate. Thus Tirek offers Twilight a trade: the release of her friends in exchange for the Alicorn magic. Despite her friends' protests, Twilight agrees to trade the magic for her friends, including Discord despite his betrayal. Upon releasing them, Tirek promptly absorbs all of Twilight's magic, making him all-powerful.
However, when Discord bestows Scorpan's medallion upon Twilight as a sign of true friendship and gratitude, it proves to be the final key needed to open the Tree of Harmony chest. As Tirek rampages through the Everfree Forest, the Mane Six open the chest, gaining a new, powerful magic that Tirek is unable to overpower. With this magic, the ponies strip Tirek of all his stolen magic and reduce him to his weakened state again. He is last seen locked in a cage within Tartarus, guarded by Cerberus.
Tirek is evil, sadistic, narcissistic, and driven by an obsessive desire for power. He has a violent temper and fierce nature, as seen during his battle with Twilight Sparkle. He is also very manipulative and highly persuasive when striking a deal. However, while Tirek may hold up his end of a bargain in some cases, he is dishonest and traitorous, not hesitating to turn on and dispose of anyone whose usefulness has expired, like Discord.
He has a deep resentment toward his brother Scorpan after his betrayal, describing him as "weak-minded" and "worthless."
Tirek is potentially one of the most powerful magic users in the show. However, his magic is severely diminished due to his imprisonment in Tartarus. His most prominently displayed abilities are shooting beams from an orange orb generated between his horns and absorbing magic from others.
Upon escaping from Tartarus, he is forced to steal unicorns' magic to restore his strength, later building up to Pegasi, Earth ponies, and even Discord himself. The drained magic empowers Tirek both physically and magically, giving him brighter colors, a more muscular physique, longer horns, and a deeper voice. His absorption powers increase in scale as well, enabling him to drain dozens of ponies at once. He also exhibits new abilities, such as creating holographic image constructs.
After absorbing the magic of Twilight's friends and Discord, Tirek's size increases to the point at which he towers over most buildings in Ponyville. His growing musculature shatters the bracers on his arms, and he grows a mass of white hair on his head to match his beard and tail. In this state, his power and magic equal that of an Alicorn magic-empowered Twilight Sparkle and he can hold his own against her in battle. Among his abilities include an energy shield, flight or very powerful leaps, and incredible physical strength and endurance. When negotiating with Twilight, he also displays abilities similar to Discord's chaos magic, demonstrated when he snaps his fingers and summons Twilight's drained friends in levitating bubbles.
After absorbing Twilight's Alicorn magic, Tirek grows from his already-impressive magnitude to a size comparable to that of a hydra. His other abilities are similarly enhanced, but he still proves inferior to the Mane Six when they gain the power of the Tree of Harmony, losing all of the magic he'd drained from Discord, the princesses, unicorns, Pegasi, and Earth ponies.