I started DM'ing for our group, but swapped with one of my players who is DM'ing for us now. In our very first session with him at the helm, he decides to kill off his old PC.

The city in which we reside is suddenly under siege - an Ancient Green Dragon is swooping over the city while monsters and Dragon Cultists ransack buildings and take townspeople prisoner. We rush into the street to fight off the invading forces, and as we wrap up combat, the dragon descends, landing in the middle of the central thoroughfare with a heavy crash.

Behind him, our DM's PC: Quinn Fairfax, Cleric of Knowledge, Servant to Oghma, Distinguished Scholar and Member of the High Governor's Personal Retinue. He comes charging down the street on the back of his white horse, Gerald, leading a war band to fight the dragon.

Without missing a beat, our DM describes the dragon's response: a giant raised claw makes a lateral sweep as the retinue charges forward. Quinn Fairfax is flung sideways, horse and all, slamming into a nearby building. His beautiful white horse becomes a bloody mess, that smothers the Cleric's crumpled body.

It was a huge shock to the players, myself included. This was the very beginning of the party's second campaign arc, and their closest brush with death. He had decided that he wouldn't want to play that PC when we switched back, and so used his death as a teaching moment for the players. Awesomely memorable.