Actual description of the achievement:
As Hades, land a single Pillar of Agony on 3 enemy gods affected with Blight.
Here we are looking at Hades and specifically his passive ability and his ultimate. Hades' passive ability is called blight which applies an additional effect on enemies when you cast your ability on a blight-infected god. For this achievement you need to hit 3 enemy gods with your basic attack (to activate blight) and then use Hades' ultimate (
or ability 4) to complete the requirements for the achievement.
It seems that you may not have to hit three enemies with blight before doing the ultimate, but what appears to be certain is that you need to hit three enemies total with the pillar of agony (ultimate ability). This will need some confirming by people who are able to earn the achievement with less than 3 enemy gods affected with blight.