Riot Axes (NA)
Why Nightfall isnt an active? Seems more fair to me that way.
If it's an active, you have perfect control over it, so anyone who likes the stats can use it effectively (Yasuo just makes sure he applies it when he has you down to ~20% health), and it's very difficult or impossible for your opponent to make you waste it.
E.g. in playtest we've had quite a few cases where you run into someone unexpectedly in the jungle and end up applying the mark by accident - which, from the opponent's standpoint, has tended to feel like "I got him to waste a cooldown".
Or more commonly, we've seen situations where an assassin will go in and their target flashes/dashes/knockbacks them after the mark is applied, causing most of the damage from the mark to be wasted. If they didn't need the damage from the mark to kill you, that doesn't help all that much, but if they did, it can be the difference between living and dying.
tl;dr: if it was an active, it'd be hideously unfair to the opponents and good on virtually any damage dealer; as a passive, it's something both the user and their target can optimize around, and which requires your champion to have specific properties in order to use well.
Also, did you consider that this may be a must buy for assasins, no matter the occassion? Im all for the item but i dislike cookie and cutter builds and the AD assasin class already has 2-3 must buy items
The item system serves a couple of very important functions. One of them is definitely choice - the Marksman item update targeted this goal - where the system should feel like something you can explore, customize, experiment with, and also have to react on a game by game basis.
A second goal is to ensure that champions feel supported - they should feel that success in game earns them stuff they care about. This is e.g. one reason why Jayce feels so bad right now, above and beyond pure power concerns - he has effective item builds but none of them really feel right.
This item is definitely much more targeted to that second goal - Assassins don't have a solid baseline of items that they can rely on, and this item helps them get there. It's possible that it's going to be mandatory on them for now, but that's still a better situation than they're in currently with the itemization system. I'd like to get them to a place where they're making tons of nuanced decisions about their items, but we're a long way out from that.