The Heart of the Wolf is an item that represents Alucard's health meter in the Castlevania: Lords of Shadow 2 Revelations downloadable content. You can upgrade the meter by collecting Life Gems from Secret Boxes.
When Dracula gave him his blood, after taking his life in a brutal fight, Trevor began a radical metamorphosis that lasted three decades. When he awoke, the corrupt inheritance of his father had taken over every inch of his being, giving him supernatural powers that cost him his humanity. Since then he has been fighting against the darkness, regenerating his wounds with the blood of his enemies and swearing to use his curse to defend mankind until the end of his days.