The Blade of the Ruined King looks ridiculously powerful for it's gold cost. So I did a little number-crunching in regards to a particular application, using Warwick as my guinea pig (Everyone's beloved on-hit champ).
In a build with Sorc shoes, Frozen Mallet, Madred's Bloodrazor, Wit's End, BotRK and a BT, Warwick's ult can do stupid amounts of damage. I'm sure someoen can think of a more damaging build, but this'll do for the early calculations.
Assuming the target is a champ with 3k health and WW is lv18 with full stacks on the BT, here's the rough breakdown of the damage Infinite Duress can cause:
420 base (+399 from AD) over the 1.8sec duration.
20% of target's life (600 in this instance) from MBR
20% of target's life (surprise! 600 again) from BotRK, but half of this is returned to WW
+210Wit's End procs
+192 total from procs on WW's passive (+16, +32, +48*3)
That's over 2400 damage, and in this instance, 80% of his target's HP before calculating resists, so stock up on magic pen runes and feast away! With a base damage of ~1220, if WW pounces an enemy with under 2k health and low enough magic resist, he can kill them outright with just the ult (if the stars align, that is).
Seriously though, that's scary as hell and will definitely see a nerf. When the servers are up, I'll try do a bot game and see how badly I can snowball some Bloodrazor/Ruined King builds.