Frome the patch notes:
Shrouded Armor:
* Renamed this item set to Witchman Armor.
* This item set now has a 3-piece bonus of Weapon Damage instead of Magicka Recovery.
* This item set now restores 28 Health and Stamina per point of your Ultimate’s cost when you activate an Ultimate ability.
In most cases this is perfectly clear, as a Nightblade, Soul Harvest costs 50 Ultimate, so this will restore 1400 each of health and stamina.
As a Sorcerer, overload requires (I think) 70 Ultimate to activate but actually costs 0 until you start attacking. Can a Stamina based Sorcerer get back 1960 each of Health and Stamina by toggling Overload without burning ultimate? On the other hand, a literal reading of "per point of your ultimate's cost when you activate" would shut overload users out of this set entirely.
The patch notes don't list a cooldown, it used to be 10sec. If it's still 10 sec then who cares; that's not enough added health and stamina to be worth the frustration of farming. If there's no cooldown and they're depending on the speed of ultimate generation to throttle the proc on this set, then it's time to farm the rift.