When you reduce a living creature to 0 hit points with a melee attack or by dealing damage within melee range, roll a WIsdom check to choose whether to use your reaction to drain its aether. If you fail, your are compelled to use it by your instinct and must use it.
You regain hit points equal to half the creature's total hit dice (rounded down) or 25% of your own maximum hit points, whichever is lower.