Zealot's Oath
Zealot's Oath is a keystone passive skill.
Percentage based Life Regeneration becomes Percentage based ES Regeneration based on maximum ES[1]. Percentage based Life Regeneration can be gained from several uniques and passives.
This will not cause Life Flasks to recover ES as they give life recovery[2].
If you have Vaal Pact, it takes priority and you will not regenerate ES[3].