1. Galactus ate four planets, so he was very well fed
2. After that he confronted four non-616 no name Celestials
3. They fought, and Galactus "killed" one
4. The three remaining merged together and one shotted Galactus
That's about it.
__________________ The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser people so full of doubts.
Who says the 616 Celestials can't merge into a super Celestial? That's silly. Just because we haven't seen them do so yet doesn't mean it isn't possible. FYI, Arishem, Exitar and Tiamut are three of the omniverse's most powerful Celestials. They're silent and deadly.
Quick question, in all seriousness :
If Galactus "killed" that one Rogue Celestial how did the other three merge with him? Because we saw once Reed REALLY killed it the Giant Celestial broke apart into their individual selves.
If he was really killed by Galactus then they couldn't form that gestalt with him.
__________________
..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
That's not exactly what happened I think. Galactus did, in fact, kill (or almost kill) the first celestial. The remaining three then merged with what was left of the presumably dead one, adding/siphoning off his energies to themselves. Considering he/she/it was already dead, though, much of what came to be the Voltron Celestial was dependent primarily on the three. That said, I don't think Reed's Sun Gun killed any of the Celestials; it certainly weakened them to an extent though. As a result, they were no longer able to maintain the Voltron-like form; and the status quo was restored to three living celestials and one dead one.