It seems there were more changes than advertised in the latest patch. The 
Varshan’s Life Steal Boss Power has been nerfed due to causing some pretty serious technical issues.
How It Worked
While the Boss Power wasn’t exactly bugged, there were some interactions with certain abilities that were causing problems. Rogues and Spiritborn had a few builds that used the power, getting some very high numbers (200 billion DoTs and similar).
The combo was with another Boss Power, 
Flesh Reaper’s Disruption, which allows you to trigger the Varshan power constantly. This happens with 
Aspect of Accursed Touch, making everything Vulnerable, and then Flesh Reaper removing that vulnerability to trigger a stun, which counts as “activating your main boss power” for Varshan’s secondary ability. This means that every single time you hit an enemy, Varshan’s power activates, creating a large Corruption damage over time effect, which stacks. You can imagine the rest with some insane amounts of DoTs stacking over and over.
The Nerf
It seems that these many, many activations caused players’ games to actually crash, while creating a lot of lag for others. This is the main reason the power was nerfed, not its damage potential. The nerf itself was pretty simple, but also pretty devastating. The Varshan boss power now has an internal cooldown. This means it cannot trigger faster than a certain amount, so it doesn’t cause server and game issues.
It does seem the nerf was warranted, but it is strange Blizzard did not include it in the patch notes. Spiritborn are particularly affected by this, as it was their best build. And considering the class isn’t doing so well at the moment, it needed the help!