If you like Nil, and want some really great Nil fics, here are some I recommend!
Bad Company (Nil/Aloy) by Nyxianthe is an incredible fic that involves Vampire Nil (which actually has a really cool, science-y explanation for the vampire curse that ties in with the Faro plague). Some really great characterization of both Nil and Aloy, some very hot smut, and a really awesome plot that weaves in with canon. (It's also going to be a series that later includes Vamp Fashav and Vamp Kotallo, but shh, you didn't hear that from me).
Saturated (Nil/Kotallo) by darkonyxdreams is a really great soulmate AU fic where soulmates see more and more color the closer they get to their soulmate. Except that Nil is born completely colorblind and believes he has no soulmate. Also will feature Aloy/Fashav!
A Warm Place to Rest (Nil/Fashav) by Every_Raindrop7 is a very fantastic, smutty fic that takes place during the Red Raids, wherein Nil has been tasked with protecting Fashav, and they go from hating each other to something more.
Scraped from Salt and Rust (Nil/Aloy/Drakka) by CthulhuButCute is a very smutty series that has some great characterization and manages to give you the feels along with the spice (the perfect combination, imo).
These are just a few of some of the really cool fics I've read by some really cool people. If you're interested in this and more, we hang out on the Nilysium discord, share snippets of our fics in progress, chitchat about nonsense, scream about Nil, and generally have a good and chaotic time. Come join us! Message me for a link if you're interested!