It’s counted as a separate game from Elden Ring proper rather than another DLC part of the main game. The thing is that, the intention is built for 3 players rather than solo: but can it be played singleplayer or not? Besides that, are there any differences between this game and actual Elden Ring (apart from the UI)?
The basic gameplay loop is entirely different for these two. Whereas Elden Ring is an open-world soulslike with huge onus on exploration and lore, Nightreign is more of a rogue-like arena-fighter soulslike. Movement and traversal are a lot faster and the focus is, more or less, on fighting alone.
Nightreign is built on Elden Ring, but at their core they’re two different experiences. If you’ve played neither, I’d suggest going for Elden Ring first, and if you like that and you also like a game to pick up and play for an hour here and there and like roguelike elements, Nightreign might be worth picking up as well.