Now they become death, destroyer of worlds

Krona- Galactus

Sure, there were cosmic, universe-threatening demi-gods in comic book fiction before 1966’s Fantastic Four no. 66. But the story that spread out into the next three issues, now known as “the Galactus Trilogy,” raised the bar. The very idea of Galactus, a being that consumes the essence of planets, knocked readers on their ear: when you take into account that the tale introduced the Silver Surfer, it’s probably the single greatest achievement of Stan Lee and Jack Kirby’s ‘60s partnership.

I’m not entirely sure, but I believe that Krona, a mad scientist from the race that would evolve into the Guardians of the Universe who obsessively seeks the secret of the creation of the universe (or multiverse), was ignored as a fecund antagonist after the character was introduced in 1965’s Green Lantern no. 40. But he was established with Galactus-level significance at the very least in the early ‘80s and afterwards.

Commonalities:
Both are shown to be present near the creation of the universe and are enmeshed in a process of destruction on a vast, cosmic scale.

Differences:
Krona is typically presented as malevolent, whereas Galactus is essentially a morally neutral force of nature.

Alternate histories:

KR: Krona is the sole survivor of the universe that preceded our own: he emerges after the “big bang” and exists afterwards as a force of nature introducing entropy to one planet at a time. Krona uses heralds, such as J’onn J’onzz, to evacuate these planets; he arrives on Earth to some years ago and is opposed by the Doom Patrol and eventually most superhuman operatives on Earth and throughout the cosmos.

GA: Galan is a renegade scientist native to the planet that would later yield the race of “Watchers”: he becomes obsessed with witnessing the “big bang” that creates the universe. But doing so has been foretold to create a process by which the universe will gradually be eviscerated. Known as Galactus in the untold billions of centuries afterwards, he is opposed first by Iron Man and the Nova Corps and eventually by most superhuman operatives on Earth and throughout the cosmos.