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At the end of Rapunzel's journey, one of her party shall turn against her
~ Demanitus foreshadowing Cassandra's betrayal
Lord Demanitus is the overarching protagonist of Tangled: The Series he was an ancient inventor and protector of Corona, initially as an unseen hero only mentioned in flashbacks but later appeared in "Lost and Found" in Vigor the visionary's body.
He was the archenemy and former friend of Zhan Tiri.
In flashbacks, Lord Demanitus is shown to be a brave and caring person who is willing to put his life in danger to stop Zhan Tiri if it means protecting Corona but later on in "Lost and Found" he is also shown to be a kind and helpful person who is willing to admit to his mistakes
In Corona myth, Demanitus was an ancient inventor of Corona who once Zhan Tiri attacked Corona created the Demanitus device to banish her and to counteract the snowstorm she made due to the Demanitus device being her machine as well.
Over the years he captured many spirits in the Demanitus Device from a mysterious event in the cosmos to powerful artifacts the Sundrop and Moonstone fell from the heavens onto earth Demanitus and three of his pupils searched for them but could not find them but then Demanitus's pupils betrayed him and summoned Zhan Tiri once again he was able to banish her again but he knew that the research on the Sundrop and Moonstone was dangerous in the wrong hands soo he split it up into 4 pieces, and then he used an ancient spell to hide in Vigor's body to stay out of sight from Zhan Tiri.
While mentioned previously in the series Lord Demanitus first appears in the episode "Lost and Found" where he as Vigor the visionary finally after over 1,000 years comes out of Vigor's body after leading Rapunzel and Eugene and starts talking to them he reveals that the reasons he has only come out now is that once he controls vigor once he will be trapped inside never able to speak again forever and he can only control Vigor for a very short time, he also reveals that he hid the last piece of the scroll inside the maze and offers to help Rapunzel, who is the sundrop, get the last piece as she already has the previous three.
Demanitus was mentioned again in "Be Very Afraid", by Rapunzel when she told Varian the time she met Demanitus in "Lost and Found" when he controlled Vigor at the Path of Demanitus. He was later mentioned again by Varian in "Cassandra's Revenge", once he learned how Demanitus had hidden the fourth incantation within the Demanitus Scroll and called the Lord a genius. Demanitus is mentionned once again in "A Tale of Two Sisters" by none other than his archenemy, Zhan Tiri, who is the one who manipulated Cassandra into betraying Rapunzel and taking the Moonstone for herself and had finally managed to escape her prison thanks to a clash between the Rapunzel's Sundrop powers and Cassandra's Moonstone powers she orchestrated. Arriving in the Demanitus Chamber, Zhan Tiri tells Demanitus he's lost since she escaped her prison and gloats that an incoming eclipse will allow her to steal the both Sundrop and the Moonstone's great powers from Rapunzel and Cassandra and absorb them into herself.
In "Plus Est En Vous", it was revealed that Demanitus tried to talk Zhan Tiri out of her quest for power, but she wouldn't listen and she accused him of turning his back on her. So, as a last resort, he reluctantly used a portal he built to send her to the lost world. Then he dismantled the device before emerging himself to Vigor, who was his pet, with a device he made. Over two thousand years later, his portal will be assembled again by Varian and the people of Corona in an attempt to defeat Zhan Tiri, only for it to be destroyed in the chaos. Regardless, Zhan Tiri is defeated for good when Rapunzel and Cassandra make amends and combine the Sundrop and Moonstone, fulfilling Demanitus' legacy.