This article contains SPOILERS for Defenders: Beyond #2
The origins of Marvel Comics' most powerful and mysterious villains are finally revealed. The Beyonders have been terrorizing Marvel heroes since 1984's Secret Wars, and even destroyed the entire Multiverse at one point before it was rebuilt into its current iteration. However, their actual origins and purpose have remained a mystery, until now.
Secret Wars was the first huge Marvel crossover that involved the majority of characters owned by the House of Ideas. An event of such scale needed an equally impressive villain, so the Beyonder was created. An unfathomable being who could treat even the mighty Galactus as a plaything, the Beyonder was later revealed to be just an infant of an entire race of creatures with power beyond comprehension, superior even to the cosmic entities of the Marvel Universe. Over the years, the most plausible explanation proposed for the Beyonders' existence was that they were "stand-ins" for Marvel's actual creatives, hence their propensity for treating the entire Marvel Universe as their playground. However, considering the role played by the Beyonders in the destruction of the seventh iteration of the Multiverse during the second Secret Wars event, a more canonical explanation of their origins had to be given at some point.
Finally, the questions were answered, thanks to Defenders: Beyond #2 by Al Ewing and Javier Rodrìguez. The Defenders, a "defense system" that the Multiverse assembles in time of need, travel back from the current Eight Cosmos to the Second Cosmos, where they meet the original Beyonder from the first Secret Wars and his peers, learning of their origins. The Beyonders were actually created in the Second Cosmos by the Celestials, who are a rogue faction of the Aspirants, the very first beings created in the First Cosmos. When the war between the Aspirants and the Celestials destroyed the First Cosmos, it also created a multiverse of infinite possibilities. To maintain order in the chaos, the Celestials created the Omega, limitless lifeforms with the power to destroy even their own creators - the ultimate failsafe in case of another Celestial war. When the Second Cosmos also reached its demise, the Celestials moved to the Third, while the Omega remained behind in the Second, to continue their maintenance duties, and took the name of Beyonders.
Al Ewing has managed to explain the origins of the Beyonders by integrating them into the cyclical nature of Marvel's cosmology, where every iteration of the Multiverse is destined to be destroyed by some great catastrophe and be reborn anew and different. The recurring description of the Beyonders as coming from another dimension beyond the regular Marvel Universe, then, is explained by making them the last survivors of the Second Cosmos, an iteration of the Multiverse that no longer exists. They occupy an empty space "outside all that is known" from where they perform their "maintenance" and experiments, including the destruction of the Seventh Cosmos. Ewing also manages to reconcile his story with the mysterious Omega Council and their Concordance Engines that were introduced in Defenders vol. 4 by Matt Fraction and Jamie McKelvie.
Defenders: Beyond also confirms that the original Beyonder encountered by Marvel's heroes was an infant, whose natural development process was hindered by an accident, leading to his interference with Earth (a story partially told in Fantastic Four vol. 1 #319 for the first time). Overall, Defenders: Beyond has clarified a lot of confusing or contradictory information about the Beyonders, and more importantly integrated these classic villains from Secret Wars into Marvel Comics' cosmology, giving them close ties with the Celestials and the Multiverse itself.