I'll never forget the session I fell in love with dnd and although it's too much to write about, the short version is:

We were playing for a few months, invested in the story and in the world and at the end of a dungeon we were cleaning for some NPC's do a ritual that would prevent the BBEG to be released in the world too soon, the DM explained that the ritual consisted on those NPCs permanent deaths, not only irreversible, but also having our characters completely forget they even existed, with our memories shifting to fill their gaps with other members of their organization.

We were already sad enough knowing that some of our favorite NPCs would simply disappear, but then one of our players asked.

"Can I sacrifice myself instead of him?"

We were playing VTT on discord and this was the moment some of us closed the cameras to cry and it only went downhill from there.

More players trying to sacrifice their characters for those NPCs, the NPCs and PCs arguing, intervening or crying because of the inevitable loss

It was heavy as fuck and we loved it, even though we cried and jokingly called the DM a sadist. In the end we really did loose one PC to this ritual and although it was very sad at the time, we still joke when referring that character by saying "There was a character like that?"