Oblivion Plague
The Oblivion Plague was a psychic disease of ultimate non-interference created by Death-Urge, but masterminded by Maelstrom as a means to neutralize the Watchers.
Maelstrom was preparing to enact his gambit to collapse the universe, but realized that a rogue Watcher might warn the cosmic entities to stop Maelstrom.

Maelstrom, able to to mask his presence from the Watchers due to his assuming the powers and station of the Anomaly spied the Watcher Otmu at his abode on an asteroid somewhere within the vast Shi'ar Empire. He noted that if he took any action even cloaked as he was, the Watcher would become aware of his presence. He was joined by the agent of Oblivion, Deathurge who was instructed to assist Maelstrom by his master.
Maelstrom surmised that within the value system that the Watchers maintained that there was a paradox. Given the Watchers Oath of Non-Interference, the idea was that even in simply observing events and not-interfering could serve to alter the outcome of events. In effect, it could be argued that even in watching, the Watchers were interfering in the events they watched. Ergo, the only way to not interfere is not to observe. And the only way to not observe was to cease to exist.
Given that the Watchers are a strongly telepathic and seemingly immortal race, such an idea would spread like a virus. Virtually every mind it enters becomes obsessed with the idea and killing any Watcher who even *subconsciously* agreed with it.
Maelstrom directed Deathurge to disseminate this "Oblivion Plague" to the nearby Watcher, Otmu as a carrier of the Plague while he traveled to Earth to begin his assault on Eon and his champion.
Later, the Stranger began discovering dead Watchers scattered throughout the universe. He had found and gathered a total of ten corpses by the time he encountered Quasar who needed him to return to his Labworld at the directive of Overmind, who had taken the planet and the Ex-Squadron Supreme hostage. Quasar agreed to first assist the Stranger unravel the mystery of what was killing the Watchers and then accompany him back to his Labworld.

The Stranger rebuffs Quasar's offer, but realizing the enormity of the issue, Quasar traveled to a nearby Watcher domicile upon a moon within nearby sector which according to Eon contained two Watchers. When he arrived, he found the pair of Watchers, one of whom was the local Watcher, named Ualu; the other was Otmu. Quasar attempted to politely ask if the Watchers were aware of that a number of them had recently died. Otmu used the sudden appearance of the unknown energy-manipulating entity to strengthen his argument that the Watchers themselves are often observed, and increased his mass in an effort to intimidate the entity into leaving their private communion.
Omtu fired an mental blast to teleport Quasar away, but Quasar merely teleported right back. Incensed, Omtu attacked Quasar with an energy enhanced punch which sent Quasar flying across the moonscape. He came roaring back with an energy-enhanced punch of his own which knocked the Watcher to the ground. They fight continued with Ualu pleading with Omtu to maintain the Watcher's Oath with Quasar getting the better of the Watcher just as the Stranger arrived as he had followed Quasar. upon a brief explanation from Quasar, Stranger recognized Otmu as a renegade Watcher and with their combined power, forced Otmu to renounce his watch, effectively killing himself.

Ualu explained that Otmu had come to explain the new "philosophical revolution": A Doctrine of Ultimate Non-Interference. Ualu also explained that already more than a billion Watchers had embraced Otmu's doctrine and had also elected to will themselves unalive.
Despite Otmu's death, the plague had continued unabated due to the Watchers' telepathic nature, to become aware of Otmu's Doctrine was to become infected with the Oblivion Plague.
Later, at the Stranger's Labworld, Overmind was battling the Stranger for revenge of past defeats when a conclave of Watchers materialized overhead distracting Overmind who was convinced that the Stranger had summoned the group to help him defeat Overmind. Stranger denied it and used the distraction to finish off Overmind.
The conclave of Watchers pleaded with Stranger in his wisdom to counter the plague with another idea, but he being somewhat jaded, agreed with the concept of the Doctrine; his words further convincing several of conclave who dropped dead. The only conclusion that the Stranger could produce was that the Watchers should become active experimenters like himself, but the Watchers could not abandon their Oath, contemplating revealing the ultimate reason for their Oath or the Stranger. Quasar sought to intervene offering a possible answer, but the Stranger disregarded him as a mortal from an young, upstart race that thought it might have wisdom that immortals such as himself and Watchers did not.
Seeking to get the attention of the gathering, Quasar flew into orbit and successfully channeled the nearby star entirely through his Quantum-Bands causing a bright solar flareup to appear just above the Labworld.
Quasar returned to find all of the gathering eyes upon him. He argued simply that the deaths of the Watchers would affect the reality more so than any of their observing might, he went on saying that even if a little subjectivity crept into their observations, that wasn't the worst outcome.

The nearby fallen Watchers return to life and one states that what Quasar had says has merit and that this new life-affirming idea would spread just as the Oblivion Plague had and that the Watcher race would continue, noting finally that humanity was wise beyond its years.