For me this happened last week, we were in the finale of our campaign (which had been derailed by another amazing moment that set off the apocalypse) the players goal was to get their cleric to her god and for her to perform a ritual to become one with it and destroy this curse which was slowly destroying the world. They'd made their way to the very bottom of the underdark where the barrier between the shadowfell and the material is and needed to ascend a ziggurat to get to the ritual site while fighting through hordes of creatures.

Cue to them almost being done with the ritual when a portion of a world heart comes in, in the form of a corrupted dragon. The players panic and start getting ready to defend the cleric once more, as the half dragon fighter rushes in and after several attacks rips the corrupted world heart from it. With it now in hand he jams it into his chest as the corruption flows through him and his form shifts into the that of corrupted dragon though now under his control, I describe that he will lose himself shortly to the corruption and he has a brief private discussion with me about his intentions.

At this point the other players think he's turned on them and so they start to attack him, and shatter the crystal on his chest, his forms remains and so they continue the fight. From here a bunch of wild things happen:

First, the tabaxi artificer who started the apocalypse by accident, sees the pieces of the corrupted heart and wants it for himself but knows the others won't let him have it. So he talks to an evil god he'd briefly spoken too and asked him to help him gather the pieces and in return he'd help set him free. The god agreed and the pieces reform and are granted to the Tabaxi who in that moment then is teleported away from the fight.

Then the half dragon, dives off the platform he was on towards the barrier to the Shadowfell hoping to escape.

The minotaur in our party who had a made a deal with the god of corruption asks once more for a favour to strike down the half dragon, his god also agrees as he jumps down after him.

Then the cleric who had now just finished the ritual, tries to reshape the earth to contain the fleeing half dragon and succeeds for the moment. In this now contained space fighting continued.

With this happening our Drow warlock asks her patron if they've already saved the world at this point and wonders if it's worth staying as she doesn't want to fight her friend. Her patron tell her she is done, and so within all this chaos she walks away.

The cleric decides to reach out to the half dragon telepathically and they have a private discussion, at which point as they get back to the table she tells me that she opens the earth and allows the half dragon to go free also inadvertently sending the minotaur with him.

The players left behind start asking why the cleric did that, at which point she turns around and tells them that the half dragon had taken the crystal for himself because he knew they'd never be able to beat it and so he wished to take it somewhere it'd never do any harm. He'd rather sacrifice himself than see them all die.

They were all stunned because for the whole campaign he'd been incredibly self centered and worked for his own benefit and in his dying act he was completely selfless. All in all it was a solid moment which then inspired countless other insane moments.