Duskblade of Drakkthar is Going to Need a Damage Cap or it Will Be Abused
http://imgur.com/y3XsTD2 [NEW ITEM] Duskblade of Draktharr Total Cost: 3250 G Recipe: Serrated Dirk + BF Sword + 850g +75 Attack Damage +5% Movement Speed UNIQUE Passive: +10 Armor Penetration UNIQUE Passive: Basic Attacks on an enemy champion apply Nightfall (120 second cooldown). Nightfall: After 2 seconds, deal physical damage equal to 90 plus 25% of the target's missing health. If you get a kill or assist on the target before Nightfall ends, the cooldown is refunded.
Instead of having the effect started with an auto attack, have it started with an active similar to BOTK. The item seems to be centered around bursty AD champions, which many of them are AD casters rather than fighters. Casters like Kha'Zix really don't get to auto during their combos which makes this item much less appealing. And yes, this would mean champions like Rengar would be broken with it. To account for this, have it so that there is a cap for the amount of damage dealt (most likely anywhere from 200-300 hundred damage would be a good cap). What I think should be considered would be:
[NEW ITEM] Duskblade of Draktharr Total Cost: 3220 G Recipe: Serrated Dirk + BF Sword + 820g +70 Attack Damage +4% Movement Speed UNIQUE Passive: +10 Armor Penetration UNIQUE Active: Marks a select champion with Nightfall (100 second cooldown). Nightfall: After 2 seconds, deal physical damage equal to 100 plus 20% of the target's missing health (damage after flat amount capped at 200). If you get a kill or assist on the target before Nightfall ends, the cooldown is refunded.
This way, a champion with, per say, 2000 health that takes 1000 damage from the opponent bursting them will take a total 1300 damage total [1000+100+(0.2x1000)] instead of 1000. From what I can see, this really can't be abused too much by assassins, but can still be used by both auto centered fighters and AD casters alike.
Another thing to take into account is champions with tank shredding abilities, like Illaoi's ultimate. Auto attacking a tank right before popping her ultimate in a fight could mean huge amounts of damage because of the damage being based on missing health. A tank with 3000 health that takes 2000 damage (I've seen it happen) would be taking a total 2,590 damage with the item's current state. A cap is most likely needed.
and other AD assassins get plenty of opportunities to auto attack, especially given how you only need to attack once per 120 seconds. in fact, it's stronger on champions that do not have to auto attack to kill targets, as it allows them to select when to activate the effect. if the passive required you to get 3 autos in a short time limit, then you might have an argument there, but as it stands you do not.