This is just a nerf?
Also the damage should cap at like 33% max HP otherwise the bonus damage at 5% HP is totally useless since you are going to kill them anyway.
Also the 100% HP damage shouldn't be astronomically low, like what the hell that thing will deal 130 damage on a target with 0 MR, on a 200 second cooldown. That is unbelievably bad.
That is an insult of an ability, it's more efficient to TP down to a lane and use a Q on an enemy then back to lane instead of using ult.
Or you could leave it like that but increase his execution damage but then you might as well make the enemy auto-open the shop when he is low and Karthus ults.