Shadow Feast advantage on it is supposed to double healing from shadow form, but it is like super ultra unreliable at that. It doubles it only once in a blue moon even if you tap or charge spam it... Imho it should just apply a buff to you after a tap for 20 or so seconds to work reliably.
"In a horror story, there are only victims and monsters, and the trick is not to end up as either." A.Wake