One of my players had a wizard who built a construct familiar and spent the whole campaign improving, achieving sentience, and befriending it.
In the campaign finale it volunteered to stay behind to manually detonate a "magic nuke" to be sure the arcane force that was tearing the world apart would be caught in the detonation. They had a limited time to decide since the storm would disrupt teleportation if it got to close and the group agonized until the very last second about it.
IRL tears were shed when they left it behind, facing the Worldstorm, waiting to sacrifice itself for them and the world.
Ashes of Athas campaign, in original run - I played a Druid of the natural world, long-term strategic goal to end the elemental intrusion into the world and bring Nature back into its rightful balance - thereby healing the currently-ruined world. One module, everybody but me was playing a pre-gen temporary character. We found a chamber where you could sacrifice to a para-element and gain its power (changing your essential nature). One possibility was Silt, but nobody else wanted to pick it. I was SO tempted, because the silt is a defining characteristic of Athas. * Ultimately, unto mine own self was I true, and stayed a Druid of Nature's Intention.
* The description was solely fluff. Mechanically, the character's race would be changed to one or another form of Genasi. I found this out much later.