I think what you're looking for is another Duskblade. But instead of dealing damage to someone after a few seconds and having about a 1:30 second cooldown, you just want it to flat-out allow someone to deal PURE damage to another champion.
Actually, I take that back. You want the old Deathfire Grasp, before Riot removed it because IT WAS BUSTED. Except this one will deal not magic, not true, but PURE damage to a champion. Do you realize what you're saying? Fountain damage is Pure damage. Fountain damage literally ignores everything and just goes straight to your health. AND it goes past passives like GA and Zilean ult. You literally cannot counter fountain damage. You can counter spell shields; just do damage that isn't magic. Besides, this new item most likely won't fit onto the build paths of many champions, making it a super situational item to build against specifically Karthus. He's not even played that much as of now.
And you're implying that Sivir's E is unfair? It's a 20-odd second cooldown shield that blocks the effects of ONE ability and gives Sivir some mana back. The shield doesn't even last for super long periods of time, and you have to time it. If having a shield that just blocks an ability is what you consider busted, then take a look at Banshee's Veil. It does the same thing, and gives some great stats like Health and MR.