I understand that concern, and I recognise that assassins definitely need to be addressed and firmly seated into their niche with the advent of League's mobility creep and the recent overhaul/buff to the ADC role (which, to be fair, was down in the dumpster for over a year). However, I don't think that this particular knife is the correct or optimal way of addressing assassins. It's honestly a really lazily constructed item.
"Assassins are having trouble!"
"Here, have a pile of nice stats so you can do More Damage, along with a passive ability that gives you Even More Damage™."
The issue was never assassins not having enough damage or enough stats. The item isn't even skill-based, since Nightfall is a passive. At least make it like a Youmu's; that is, a temporary empowerment that a player must remember to use and must be clutch about using. Honestly, the new assassin item feels like an unnecessary amount of babying to assassin players---something that many of them may not even want. I know several assassin players who play the role specifically because of the high-intensity, high-skill, one-in-a-million assassinations. It'd actually take away from the identity of good assassin players to have an item introduced that's a pile of delicious stats and a Free Extra Damage passive.
Assassins are supposed to outsmart you. They're supposed to pick the most opportune moments to make one game-changing move---whether it be by catching you completely off guard, or by crashing down on you when you dread it most. Assassin players should be made to weave through the thick of battle to find their marks; indeed, that's what makes good assassin players like Bjergsen, like PawN, like Faker---that's what makes them incredible to watch: their intellect, their precision, their skill.
tldr if there's to be a new assassin item, it should be heavily tied to skill, and it should most definitely not be a candy bowl of Stats Players Want.
