Huitzilin, AKA the Left-Handed Hummingbird
Although Huitzilin had been introduced as part of the ‘Alternate Universe’ arc- where The Doctor was tracking a foe who was manipulating his own timeline for unknown reasons , he would be a unique and interesting villain even without his role in such a significant storyline, due to his complicated history and his unique connection to The Doctor.
Essentially was a psychic vampire, Huitzilin, an Aztec warrior born with a clubfoot, who was exposed to the energy of a ruptured Exxilon fuel pod that was somehow acquired by his tribe while they were travelling only to be abandoned in a cave where Huitzilin and his men took shelter. The radiation mutated his DNA, turning him into the most powerful psychic the human race had ever seen, not only prolonging his physical life but learning how to manifest his spirit after death as a being of pure psychic energy.
However, his post-death state was fundamentally unstable, requiring him to draw on the psychic energy of others – typically released by human sacrifices – in order to keep going. Under normal circumstances, Huitzilin’s psychic energy would have eventually dissipated as the energy required to sustain him grew so powerful that the Aztecs would have lost the ability to provide him with it no matter how many people they sacrificed. However, due to the manipulations of his initially-unknown foe, the Seventh Doctor was tricked into a position where he would be directly bonded to Huitzilin, thus providing Huitzilin with a surplus amount of energy due to The Doctor’s natural power as a Time Lord.
In this state, Huitzilin also gained power by feeding on death and other powerful emotions; he fed on the deaths of the Aztecs at the hands of The Conquistadors, the disappearances in Chile, the famine in East Timor, the China/Vietnam war, the civil war in Rhodesia, and the terrorist actions in Brazil, and engineered John Lennon’s assassination to feed on the grief of Lennon’s fans. He also became ‘time permeable’ in this state, allowing him to know at least some aspects of the future, such as him becoming aware of his future encounters with The Doctor when they met each other for the first time. Due to the connection formed between the two, Huitzilin began to draw on The Doctor’s existence so that he could be reborn into the physical world. Aided by Cristian Alvarez – a man who had experienced Huitzilin’s psychic force in his past and their future, Cristian accidentally calling the TARDIS for help in 1994 before they had met him , The Doctor, Ace and Bernice Summerfield were able to track the psychic force back to its Aztec origins, but were unable to stop the sacrifices that allowed Huitzilin to ascend to his non-corporeal level as it was a part of history, his visit to the past also allowing Huitzilin to bond with The Doctor when he tried to put himself into a trance to learn what he was up against.
Travelling through time to track down the earlier manifestations of the Blue – the representation of Huitzilin’s psychic force, attracted to the younger Cristian due to his psychic potential and past experiments with LSD – after Christian was incinerated by its power in 1994, The Doctor was able to identify his enemy, but their attempts to fight Huitzilin by tracking his appearances throughout history only allowed Huitzilin to manifest in those times and participate in various atrocities. The TARDIS crew eventually managed to trace Huitzilin to the Titanic – although they had been forced to materialise on the ship on the night it sank to limit their potential historical impact , attempting to trap him in a copy of an Aztec Codex that went down with the ship, only for The Doctor to be unable to complete the ritual as he and Huitzilin were now too closely tied together, The Doctor unable to stand the pain of separation. As Huitzilin mocked The Doctor’s affection for humanity, informing him that humanity had committed all of the crimes that he had fed on without his prompting and all he had done was benefit from their actions, he achieved physical manifestation, only to be show by Cristian, the surprise of the attack fatally wounding Huitzilin and allowing them to defeat him. The TARDIS crew departed, Christian rescuing a baby from the sinking Titanic as they left and vowing to make something of his future, undoing the death he had previously suffered in the travellers’ ‘first’/last encounter with him in 1994.
The enemy responsible who arranged for The Doctor to be bonded to Huitzilin would eventually be identified as the First Doctor’s enemy The Meddling Monk (“The Time Meddler”), who had allied himself with the Vardans (“The Invasion Of Time”) and was using a Chronovore (“The Time Monster”) to reshape history to distract The Doctor while he helped the Vardans prepare to invade Earth in revenge for their previous defeat and betrayal by The Doctor and the Sontarans respectively (“No Future”).