Wrathia's Heart Key is collateral given to Ava Ire. It was created from Wrathia Bellarmina's heart, when Ava asked for a piece of her soul in order to ensure that the pact would be completed. Wrathia gave Ava her heart because it is the piece of her soul that she claims she no longer uses.
The key ensures that after Wrathia's half of the pact is complete, she cannot separate from Ava until she properly fulfills her side of the pact; giving Ava a new, completely different life.
The key first makes an appearance on page 0295.
Besides being able to open and lock the Drawer of Wrath, the heart key has no known powers. Even so, without it, Ava and Wrathia cannot complete the pact. A failure to complete the pact would cause their merged souls to become a "monster of failure".
The key has the power to turn into a battle axe form. In this form, the key grows several feet in length, and is summoned when Ava pulls it out of her chest drawer after drinking the red vial.