Aku

Aku is an evil shape-shifting demon from the tv show Samurai Jack. He is Jack's mortal enemy and he is obsessed in destroying the young samurai.

Backstory
Years ago in Equestria when Cadance was a foal, a group of equines unleashed a break evil know as Aku! And he unleaded a mortal evil across the land, when Princess Celestia and Princess Luna confront him but they were now match for the demon. And he banished the 2 far away, and he takes charge. While banished on a mountain they meet the three gods Ra, Vishnu, and Odin who explain what Aku is and taking the pure spirit from the, the gods in the form of three monks forge a enchanted sword.

And there manes change making them older, the 2 returned to Canterlot when Celestia sees a sight of Cadance in danger. They confront Aku and explained the sword for Aku to respond with a simple BAH and started to blast at him with his eye beams, dodging the beams or deflecting them and slashing Akus form again he roared in pain and turned into a flying dragon and chased after him but before he could eat the emperor he slashed his head off and fell down , Aku then morphed into a spider and charged and managed to knock the emperor away. Then Aku exploded in a rain of black goo of his substance and took the armor of fallen warriors and became a whole army first laughing insanly, then glaring stupidly, and then they attacked. Each one was cut down by the emperor until one was left now looking scared. As Aku fled in terror the emperor thrust the sword into him and morphed him onto it, he then shoved the sword in the ground and Aku is sunk into it and is then turn into a chamber.

The Princess returned to Canterlot and see Cadance crying but unharmed. And soon everything was restored.

In Brian Griffin's Adventures of Transformers: Age of Extinction
Meanwhile in Canterlot, a unicorn stallion walks through an alleyway while levitating a bucket of oranges. He encounters black slime he kicks a rock in it and soon a massive creature with giant horns, flaming eye brows, giant teeth, and a body made from a black void of emptiness emerges from the pit. Aku (Japanese for "Evil") has entered our world. When Aku notices the unicorn stallion on the ground he thanks him for releasing him. And he claims he has unfinished work to do.

Later after Cade discovers Optimus, Princess Celestia gets a sight on Aku. And she and Luna explained about him. Then later he joins Bowser and in the battle of Hong Kong, he notices the Princesses, they were prepare for this. And they battled the great evil. Aku is almost earned victorious, when Cadance fired several shots at him, b ut he quickly retreats. Then he is killed in the train chase.

In The return of Aku
Aku is back, and he's badder than ever. First he rallies remaining Deceptitrains, the he goes to Tartarus and kills Cerberus. And frees all of the prisoners, including Evil Jimmy. And he plans to make a giant machine filled with Cybermatter and poor it into the largest volcano and flood Equestria and Equinelantis with lava. After knowing Aku has returned, Princess Celestia tell Jimmy and his friends to spy on Aku and see what he's up to next.

They found out soon enough, they saw that Aku's evil plot is starting. Then Evil Jimmy captures Skyla and he tells our heroes that Aku is saying,, "I'll need something else in return for your cooperation" and then he leaves. Our heroes are brainstorming a plan to get Skyla back as Mako's trying to tell them something. Then Double-D finally came up with something, he and his other 2 friends sneak into Aku's fortress. They soon find Skyla under guard of some of the Deceptitrain and Jimmy, they manage to drive them away from Skyla's prison and sneak inside. Then they switch Skyla with a decoy one with a body made out of a vacumm, paper wings, and a head made out of a pineapple.

Back at Canterlot, Cadance and Shining Armor embrace Skyla. Then Jimmy explains what Aku is planning, after Aku knows Skyla's been freed he arrives at Canterlot castle and attacks, during the battle Jimmy kills his evil clone in a fist fight. They took cover at Equinelantis. King Solar Flare decides that they need to kill Aku before he gets his chance. Our heroes attack and kill the Deceptitrains and Tartarus prisoners. Thomas, Twilight, and the Princesses battle Aku. Terence then jumps and aims the Elemental sword at Aku but then he fires 3 blasts at Terence, then everyone watches in horror as the blasts hit Terence and dies. And then he falls into the pool of Cyber matter! Then Thomas, Twilight, and the Princesses angrily beat up Aku, but before they can finish him, Aku throws a peice of metal at them, sending them dangling over the side, right above the volcano! Aku then takes a sword and prepares to make them fall into the volcano, but then Aku stabs Aku and falls to the volcano and dies. They took the Cybermatter to Canterlot and guard it for all eternity and Optimus sends out a message.

Relation with Tirek
In Brian Griffin's Adventures of Transformers: Age of Extinction, Aku joins the villains after he was free and he made a relation with Tirek. And they quickly made an alliance. And then later, the centaur and the lord of darkness were firmed allies.

Trivia

 * Aku is voiced by the late Mako Iwamatsu.
 * Aku is also Tirek's partner.
 * Aku is also one of Bowser's partners due to that he's the Master of Darkness.
 * Aku became the antagonist in The Night to Remember.
 * Aku is one of the few Cartoon Network villains who has actually taken over the world. However, in the Pooh's Adventures trilogy he is not ruler of the world.
 * Aku returns in Brian Griffin's Adventures of Transformers: Age of Extinction.
 * Aku will become Ash Ketchum's new enemy in Ash Ketchum Discovers Laputa: Castle in the Sky.
 * He then returns in and Age of the apocalypse but is killed but the Terminator and the foals.