CactusDragon517 from theoldguard-recs here! I’ve done some digging on AO3 and found two that might be fitting what you’re looking for for this. No worries about dark - sometimes you just wanna read sad stuff, right? (Or is that just me?)
Nicky knows his time is up in the most innocent of ways, he stubs his toe against the table in one of their safehouses and it doesn’t immediately stop hurting. Instead, it hurts for hours. His suspicions are confirmed when he cuts himself in the kitchen the day after and the wound doesn’t seal the way it usually does. He hides in the kitchen, cooking for hours until it stops bleeding and he can pretend it isn’t there.
Warnings: Major Character Death
Additional Tags: Angst with a Happy Ending, Nicky loses his immortality before Joe, But they still go together in the end, Moments of a happy life, Found Family, Their Cottage in Malta
Why I Love This: This was the very first fic I thought of when I saw your ask. I think that it is soft and sad and sweet all at the same time - it addresses just how it might affect all of them and Nicky himself. Nicky’s voice is SO true and understated in this, it’s a real gem for me.
el_gilliath has written one other fic for The Old Guard, though I could find no mention of their tumblr on AO3!
And my second rec might be a little unconventional, but it’s Your Work Wednesday, right? I’m reccing one of my own fics!
They find out later that the young man’s name was Lykon.They find out that should they lose the gem, they will not rise again. With the gem, they can heal from most wounds, but once the gem is gone, so is their immortality.It is hundreds of years past their first awakening, Nicky curled around Joe as he comes back to life, slowly but surely. The gem in the hollow of Joe’s neck winks cheerfully in the sun and when he draws his first breath back, Nicky covers his face in kisses.
Or, a different look at immortality and The Old Guard.
Warnings: Major Character Death
Additional Tags: Universe Alteration, Alternate Universe - Canon Divergence, Immortality in a vessel, this is sad and I apologize but I can kind of promise a happy ending?, Tumblr Prompt, background andy/quynh, selective mutism, Nicky goes first, but it works out in the end, Sort of Reincarnation
Why I’m Reccing It: This is kind of a weird one, I admit - it’s sort of adjacent to your prompt in a way but I thought I would take a chance and put it out there. I’m proudest of creating this universe alteration as well as why Nicky would make that choice. Also, I really love exploring what the distant future might look like for The Old Guard crew.
I’m obviously on tumblr and have written more for The Old Guard - you can find me on tumblr at CactusDragon517 or on AO3 as MayQueen517
Aside from these, I couldn’t think of any others off the top of my head so I happily invite anyone else to add any on they can think of!