This is how Ekimu tells the history of him and Makuta to Princess Celestia and Princess Luna goes in The Return of Makuta.
[Ekimu walks over to Celestia and Luna]
Ekimu: It is so sad how evil my brother has become.
Princess Celestia: Did he turn into an evil alter-ego of himself?
Ekimh: Well he turned into a massive shadow titan of himself, but he kept his name.
Princess Luna: Really?
Princess Celestia: So Makuta wasn't always evil?
Ekimu: No. He was jealous and ambitious.
Princess Luna: And what made him so jealous and ambitious that he became evil?
Ekimu: I suppose that is where evil begins with ambition.
[A flashback to the past begins]
Ekimu: (narrating) The masks we made shaped the lives of those who wore them, but I never realized how badly my brother longed to outshine me.
[We see Ekimu walking into the building and Ekimu sees Makuta merging two elements into masks]
Ekimu: (narrating) So Makuta broke our most fundamental laws and merged elements into masks. He hoped by making mericals, Okoto would see that he was more than just a simple Mask Maker.
[Makuta gives one mask to a groundskeeper and he takes off his mask and puts the mask Makuta offered him on and touches a tree and the tree grows fruit. He takes the mask off and gives it back to Makuta and Makuta gives him another mask and the groundskeeper puts it on and touches the tree again and its leaves burst into flames. The groundskeeper takes off the mask and gives it back to Makuta and puts his own mask back on]
Ekimu: (narrating) But combining even two elements in a single mask brought disaster. Makuta should've stopped, but he didn't.
[We see Makuta in the building merging all the elements together, creating the Mask of Ultimate Power]
Ekimu: (narrating) So hungry was my brother for power, that he merged all the elements into one Forbidden Mask.
[We see the capital city of Okoto. Makuta presents his new mask to the people]
Ekimu: (narrating) He took his new mask to the capital. Makuta hoped the power of his new mask would allow him to rule the planet like a god. But I knew it wouldn't be so. Prophecy had foretold it all.
[We see Ekimu, with his Hammer of Power, running to Makuta. Makuta takes off his Mask of Control and puts on the Mask of Ultimate Power]
Ekimu: (narrating) My brother had been pure heart, the power but affected him differently, but the mask's powerful alchemy turned my brother's envy and bitterness into pure evil.
[We see Makuta's face getting dark energy around it]
Ekimu: (narrating) I did what I had to do.
[Ekimu reaches Makuta and uses his hammer to knock the mask from Makuta's face]
Ekimu: (narrating) The power of the mask tore a hole between our world and the Shadow Realm. The city was drawn into that realm and so was my brother. And then it was gone and our world was as you found it as it is now.
[We see the city and Makuta being pulled into the Shadow Realm and nothing is left but debris and Ekimu]
Princess Celestia: Oh, my.
Princess Luna: It's a bit like me when I turned into Nightmare Moon.
Ekimu: What is saddest of all is that it was all foretold in prophecy.
Princess Celestia: I can tell.
Princess Luna: Sounds similar to what I have endured before.