I actually REALLY like the game and I'm currently working on 100% it. It adds new mechanics(although it also takes a couple away but it wasn't a huge issue, you'd just need to adapt) and new achivements as well Nightfall is basically a prequel to Aragami in a sense that it explains how certain characters came to be where they are in the beginning of the game(not sure if you care about spoilers so I'm keeping it vague) I'd say it's worth it if you like Aragami Base Game. Yeah it's co-op(both Base Game and Nightfall) but you need two copies of the game(at least for Xbox) & Xbox Live/Gold

It definitely focuses more on stealth than combat(it's definitely a more Mark Of The Ninja type stealth than say a Tenchu action game, it's definitely not a Dishonored Play-Your-Way type of stealth). At first I hated it mainly b/c I wasn't able to carry bodies/lure away guards so I wouldn't get caught/explore the area more thoroughly(you do gain that ability a couple of chapters later) then the game became WAY more fun if you love stealth games.

Aragami nightfall cheap