When the renegade Malthusian, Krona, attempted to access records of the beginning of his universe to learn its secrets, he unleashed waves of energy that spawned the Multiverse. In addition to infinite positive universes, there was also an anti-matter universe and twin worlds in the prime universe and its anti-matter counterpart known as Oa and Qward. Also spawned from this act was the creation of twin Monitors, two twin beings that would be instrumental in phenomenal events yet in the future. While Anti-Monitor created his warriors, the Thunderers, to faciliate his conquest of his universe, the Monitor of Earth-One's universe resided for an unspecified period on Oa's moon in solitary contemplation. Meanwhile, splintering occurred among those from Malthus, as these decided to leave the former brothers the Guardians of the Universe and their native dimension behind. Therein, these individual Controllers each ventured into a "segment of the multiple universes" (please see clip attached) i.e other universes such as Earths Two, Three, Four, X, S and others. As the Controllers, each member of this powerful cartel supervised their own assigned dimensional planes of existence, operating in much the same capacity as had the Monitors in their respective universes. During the eons that the Monitors lie immobile after their lengthy conflict, the Guardians of the Universe and the Weaponers of Qward filled the void of oversight in their respective universes, as had the Controllers in the remaining universes... for billions of years.
When the Monitors had been revived, the Crisis on Infinite Earths begun that would bring all these entities into decisive conflict. And a thousand years in the future, in a timeline wherein the Crisis did not remove Earth-One and its parallel planets, one of the Controllers returned to his home universe (left vacant after the loss of the Monitor at the hands of his twin in the 20th century, although for a brief period his position was filled by his heir-apparent in Lyla Lerrol Michaels aka the Harbinger, who possessed a portion of his powers for a brief period of time). This Controller attempted to clean up what he perceived were flaws in his native reality, using a Sun Eater to accomplish this, and perhaps previously resided in Earth-Three's corrupting universe. After being defeated by the Legion of Super-Heroes, he perished and a brother of his briefly visited their realm to reward them for their heroic act of valor, perhaps of neighboring Earth-Two's universe. A third Controller assumed the alias of the Time Trapper, ensnaring the inhabitants of future centuries in the grip of his control before the Legion upended his schemes, assuming the vacancy left behind by Monitor's death. Eventually, this Controller's powers were sapped by Darkseid of Apokolips, never to return. As for the Controllers in other universes, these mainly operated behind the scenes overseeing their assigned realms as had the original Monitor, whom they modeled themselves after. In the timelines wherein the Crisis had not merged all Earths into one, it was due in large part to each Controller who protected those futures from assimilation. Thus, just as Earth-One's Legion future was preserved and tied to its 20th century Superboy, so too was Earth-Two's Brane "Batman" Taylor of the 30th century connected to the 20th century Batman, and Earth-S' Billbat tied to the 20th century Captain Marvel. And this just scratches the surface of timelines shown.