if a champion needs to proc its effect multiple times on a champion to be able to assassinate someone then they should not be assassinating them out right
the proc is supposedto help them kill someone, not be a 50% damage increase
the item proc should never be dealing 50% of someones total hp at any point of the game, yet kha can proc is 4 times on a champion in about 5-6 seconds for 1280 bonus physical damage at lvl 18, and many of the squishier targets like adc, supports (non tank), and mages barely get close to 2500 hp at level 18, the change im suggesting would make the item more fair for all designed assassins and leave room to buff it more in other ways
for example if it went to the 9 second CD at lvl 18 per champion on its proc it could gain a little more base damage, example instead of the 65-320 (going off this websites item info) it could be 75-400 AND causes the target to be vulnerable for 3 seconds
vulnerable targets are 30% weaker to your lethality for 3 seconds (meaning all your total lethality is more effective on them for 3 seconds
so in effect if you have
+ any other source of lethality would be 130% effectiveness instead of 100%, this would in turn let duskblade still offer its burst damage, but keep its effective damage more in the champions abilities which would be more effective due to higher lethality effectiveness for the 3 seconds after procing duskblades effect
AND this offers the counterplay option of CCing the assassin as he jumped his target and making him lose out of that damage bonus instead of him just reprocing the item instantly and recking some more, makes escapeming the assassin more rewarding even if its a temporaty escape, makes dodging his abilitires more rewarding.
this would make it more of a skill factor than a right click and win factor after going out of vision for a second, effectively reopening outplay potential and making it more fair, while still providing the function the item is meant to have