This hasn't got much to do with lore but is a story about probably the best moment I've had as a DM.

I've recently been running a fairly adapted Wild Beyond The Witchlight campaign with my in-person group. We were getting to the end of a four hour session, and the party had reached the beginning to a dungeon, but decided that because it was nearing the end of the session and we were missing a player, it would be best to leave entering the dungeon until the next session and instead go on a side quest to fulfil whatever wacky shenanigans they had in mind. Fast forward about 10 minutes of play later and one of the players characters has ended up in Wizard Jail, because my party have a tendency to go murderhobo sometimes. The player asked if there he had a cell mate, and I thought 'Sure, why not, they're not going to be important anyway'. However unlike other NPCs I come up with on the spot, this character appears in my mind quite clearly, being a Chaotic Good high level wood elf chronurgy wizard with bronze skin, short brown curled hair and yellow eyes, that could easily escape the prison but chooses not too, and got arrested for acting as a robin hood like figure in the community. So I outline this character to the party and one of the players asks the dreaded and fated question: 'What's his name?'. 

It had been a long session for me, so I decided this character was called Rizijimm, pronounced 'Rizzy Jim' (Yeah, I told you it had been a long session, the character names had reached brainrot levels). So my party chuckles at the name, and when the player in the cell is picked up by a guard to be sent to trial, the player takes the opportunity and fights back. Ensue and epic fight scene between the mage-guard and the player, alongside Rizijimm, who the guard directs most of his attacks towards (the in-game reason being that the guard is aware that Rizijimm is stronger than the player, the out of game reason being that I didn't want the hassle of the player having to heal up much before moving into more of the prison or god forbid getting downed without any healers around). They both manage to take down the guard, and my players had really started to take a liking to Rizijimm, although he was heavily injured. Then, more guards started appearing, and attacking, trying to avenge their fallen comrade. My player trys to fight against them but is easily outmatched, and so, in his last breaths as he's attacked and killed by the guards, Rizijimm uses his high-level Chronurgy magic to turn back time by an hour and send the players all the way back to the entrance of the dungeon, sacrificing himself.

I know some people would think that's a bit of a cop out and bad DMing, however if you've played or DMed WBtW, you'd know that time magic is a strong part of it and treated differently to regular DND, also it was very much nearing the end of the session now at this point, and I wanted the party to have made it back to the dungeon's entrance ready for the next session and so the player that had missed out this session wouldn't be as confused, and at the pace they were going and how long we had left, they would have never been able to make it back in time. However, that's beside the point and doesn't impact the story, so doesn't matter. After Rizijimm's noble sacrifice, my party were devasted, like actually in pieces and grieving. I had killed of NPCs that were far more significant in the past and they didn't care even a fraction as much as they did now. They were begging me to bring him back, to do something to save him, so much so that the last moments of the session were just spent lamenting about how great Rizijimm was and how much they'll miss him (they knew him for the better part of half an hour max) They all left having really enjoyed the session and it really made me feel like I had managed to achieve the level of DMing I had wanted to throughout all the campaign, having managed to make such an impact on my players even though it was over a stupid, insignificant, thought-up-on-the-spot NPC. My players were scheming on how to bring him back or feature him in future campaigns, but all agree that it would have less of a narrative impact if they did, so now just treat him as an honourable and incredible sacrifice, and the greatest wizard they had ever known.

RIP Rizijimm, fly high

The best characters are the ones who were not even meant to be important. Its like the characters single out the one random NPC and get weirdly attached. Players are weird.

I'm happy you had that great DM moment!