So the item says that you gain it's passive if you are unseen for 1 second. But if you attack minions or monsters and get unseen you won't get it. You can only get it by being in combat with an enemy champion and then get unseen. It works fine if you proc it on champions and get unseen then though. I played Kha'Zix and didnt get Duskblade passive when entering fog/jungle/bush after attacking minions/monsters.
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