Here's my thoughts.
Assassins lost Last Whisper this season. All they gained was 10 flat pen on Maw which is a situationally built item. So, assassins essentially lost some damage even vs squishies this season, because 35% of base armor even against squishies was more than 10 flat pen from Maw. This is a way to bring their damage back a bit. This isn't an item to kill tanks. This doesn't mean tanks will get instakilled. It just does a good deal to them, like last whisper used to.
Even though tanks are supposed to counter assassins, it's unfair that assassins have no options vs a tank really. It gives an option that has more than one use, that being killing squishies a bit as well.
However, this is a passive cooldown item. That is its balancing point. Every 2 mins, you get this proc. And it's cool if you're going all in for the kill, but anyone that says the only time they've auto attacked another champion is in an all in is lying for the sake of demonizing this item. There will be many MANY situations where this item has to be wasted and you won't have it when needed. If it was an active then it would be completely OP.
Also, I think it gives context to nerf Zed. When you look at what Zed is designed to do, where he acquires his power curve and where it ends, then take into consideration the various items and summoner spells in the game as well as popular champions played, you'll see Zed is actually in a good place. He really always has been. But, if people complain enough, they'll look into nerfing something. This item is a good way to give reason to nerf Zed in that with this item and NO balancing, he will be straight OP, since the mark synergizes with his mark both if it procs before his ult or after.
That's just me though.