For some reason the lvl 54 Call of Darkness spell (Drains HP and gives +ATK) is not stacking with the Vampiric Curse Recourse (Increase HP per tick). The Recourse just overrides darkness like it is a more powerful spell of the same type.
Call of Darkness is not a +hp its a -hp. Its just like the necro spells that drain hp over time for mana over time, except it drains hp over time for attack. Vampiric Curse is a damage over time life tap.