; Date: February 12, 2020
In a speech to the Doctor -- why do villains always give speeches describing their play? -- Zellin mentions other immortal beings including the Celestial Toymaker (from the First Doctor serial of the same name), the Guardians (from multiple serials in the Fourth and Fifth Doctor series), and the Eternals (from the Fifth Doctor serial Enlightenment).
Zellin and Rakaya, in a comic-art interlude, describes the cosmology of their situation:
Two creatures from another realm descending into a universe where they were worshipped as Gods. They saw two planets, and made a wager. Which of them could bring a planet to destruction first.
The Gods set to their games, sowing chaos through the populations. Wars began. Between species. Then between the planets themselves. The Gods delighted in the carnage. It passed the time.
But, slowly, the inhabitants of the worlds grew wise. They realized what these creatures had done to them. They unified and fought back against their so-called Gods. They set their own planets into a collision course. And at the heart of the collision they made a prison. They trapped one creature between the planets for eternity. The other fled, vowing to return to release his eternal partner.
This described the story of Zellin and Rakaya. For the other God-Immortals the Doctor has encountered, the stories were different but there was a core similarity between them. Namely - there were a variety of beings that existed before the Universe was created. Some of those beings survived into modern times.
Some of these God-Imortals were benevolent or even kind. Others weren't, and were interested in things that caused harm to the mortals. But the lives of the mortals weren't of much concern since mortals have such short flickers of lives.
The Celestial Toymaker
We met this being in a First Doctor story of the same name, aired in 1966. The Celestial Toymaker was described as immortal, and had been cast out from another universe, and occupied his own universe. The zillions of years of isolation had driven the Toymaker insane.
To relieve his boredom he cast a net to capture beings who would be put through games/contests. If the Toymaker were to lose a game, his world would be destroyed and he would then rebuild it. If the contestant loses, then the contestant is added to the world. If instead the contestant wins, he is destroyed along with the Toymakers world.
Basically - The Toymaker uses his power to bully and entrap others. Why? Boredom.
The White Guardian and The Black Guardian
The Guardians are a pair of such God-Immortals who are locked in an eternal contest. Basically, they served to personify Good and Evil, and the continual battle in people between their Good and Evil nature.
The Guardians ensnared The Doctor in several episodes. For example one whole season was devoted to finding the six segments of The Key Of Time. The White Guardian and The Black Guardian both sought to have control over the thing, and The Doctor and Romana were sent on the quest to assemble the segments. Once the segments were assembled it was determined they had the power to control all time everywhere.
In another appearance, the Guardians were present in an episode involving another set of God-Immortals, The Eternals - so we'll describe that in a minute.
The Guardians weren't acting out of boredom. They were in a real contest with each other, always seeking to win over the other.
The Eternals were a race of God-Immortals who were bored and staged a contest among each other in a series of Fifth Doctor episodes called Enlightenment. In the contest each group of Eternals put themselves on a spaceship with characteristics of Earth sailing ships. One took the shape of a 1800's British sailing ship, another a Pirate ship, another a Byzantine oar-powered ship, and so forth. But they're flying through outer space, and there's a bunch of technological wizardry.
For crew - The Eternals kidnapped sailors from Earth, and also had them mentally controlled so the crew wasn't all that aware of the oddness of where they were and what they were actually doing. The Eternals used the word Ephemerals to describe the humans, such as their crew, and really didn't care about the fleeting lives of these Ephemerals.
The Guardians were involved around the edges of these episodes as well. One of the Eternals was working for The Black Guardian, as was one of The Doctor's companions.