Immortality has been a pretty common character element since basically the beginning of storytelling. You had monsters who were immortal, humans who were going monstrous things to become immortal, and beings like elves and angels who were ageless or outside of time. I hadn't really thought about it because it so baked into the core of many races and creatures.
And then I read Orconomics. It was a very enjoyable read, with a Dungeons & Dragons inspired setting that focused on the various economic forces and motivations that would be in place to create a Dungeons & Dragons-esque setting (more details here and here if you don't mind spoilers). But the real treat for me was how his elves remembered their past. But first I need to go on a tangent, which will loop back to this, I promise.
Various neurological discoveries made by scientists tell us that, in summary, the human memory doesn't last and isn't reliable. Your brain will "delete" old memories to make room for new experiences, which is why people usually only have vague recollections of their childhood as they get older. And every time you access a memory, your brain might "overwrite" the original memory with your remembering of the memory, and every time your access that re-memory it might overwrite that with the new re-memory. You can, in fact, make up entirely false memories and then keep remembering them until your brain forgets they are false and make those false memories so sensory rich that they seem more real than your actual created-from-real-events memories.
I had been vaguely aware of these various facts for a couple of years, but Orconomics was the first book I read that incorporated these discoveries into his fictional world. Pike world-built a culture around the facts that the elves were immortal and that their memories weren't. His elf main character, Kaitha, was a washed-up drunk with a developing drug habit who had been an amazing hero. There were stories and legends about her deeds, of the cities she had saved and the monsters she had defeated. And she couldn't remember any of it.
She still had her skills, but she couldn't remember who she had been when those stories were started. Why had she fought those monsters? Why had she started drinking? Why had she become a hero in the first place? Enough time had gone by that all her formative memories had faded away, and all the external touchstones she might have used to remind herself had been replaced. She had moved on with the world around her, and the person in those stories was a complete stranger to her.
In Pike's world, his elves lived in one of two ways. They either grew on, essentially becoming an entirely new individual indistinguishable from any other modern person every couple of hundred years, or they purposefully repeated their life, freezing their surroundings and habits. Kaitha and her sister were examples of the former choice, and his examples for the latter choice was the Queen of the Elves and a specific elf couple known about by Gorm, the protagonist.
In the Queen's case, she never left her home and she always had three children, with her grown set of children being stripped of their royal titles upon the births of her next set of children. Nothing ever changed about her home or life, with everything being arranged to reinforce her memories and personality. In the case of the mentioned couple, the two elves live in abject poverty, working to save up enough money for an elaborate wedding. And once they have the wedding, the love-struck pair separated to repeat the cycle all over again.
I found this all fascinating. It gave me so many ideas of how this possibility could be used in other ways. What if, instead of forced repetition, the long-lived race had rituals and traditions built into their culture for the purpose of reinforcing memory? How would this race use sensory memory? What if they forgot, but made sure to write down everything that happened to them for the purpose of reminding themselves? What about elaborate manner codes to give their culture a sense of stability? What would they think of short-lived races?
What if the monster eventually forgot why he or she had become a monster, or even if they had ever been anything else? What if a being isolated themselves for a long period of time and forgot about the other species? If a race had a "racial memory" could it be trusted to be accurate? How did they experience that memory in the first place and how would that affect how they felt about said memories?
I have since found traces of these elements in other media, such as League of Legend's new Vladimir lore, and I look forward to finding it in other works!