A math. proof that Hexdrinker is broken and shouldn't be allowed to coexist with Nullifying Orb v2.0
Syndra:
(fully stacked)
AP: 102
Armor: 96
Magic penetration: 18
Zed:
AD: 145
Magic resistance: 77
Lethality: 12
Raw 6 spheres Unleashed Power damage: 662 magic damage.
6 spheres Unleashed Power damage after damage mitigation without Hexdrinker: 536 magic damage.
6 spheres Unleashed Power damage after damage mitigation with Hexdrinker: 417 magic damage, - 200 from the lvl 10 shield and -90 from Nullifying Orb = 127 magic damage.
The close peak of Syndra's R burst was close to being completely denied from a single rushable item and a rune. 536 magic damage -> 127 magic damage.
I had a fully stacked Keeper's Armguard, for 45 armor. This item increases my physical damage resistance by 15% (33%->48%). Is it even close? I DO NOT think so.
Hexdrinker is broken for the assassins. It should not be allowed to stack with Nullifying Orb. Hexdrinker is way more cost efficient than Seeker's Armguard.
If the situation was reversed, i would have died to his burst. This is totally unfair and biased.
Couldn't land enough abilities to burst?
Don't have enough mobility to roam?
Played with impunity and didn't respect mages' damage? 

1shot everyone lul.