Here's my idea for the spell:
Astral Souls are gained by casting Fire 4 when you have no Umbral Hearts. Meaning during your opener, when you'd get four before the Despair, you'd have enough for a full power Flare Star. The Astral Soul would show up on the same place as the Umbral Hearts, just colored red.
The spell would replace Despair's button, and act as a direct upgrade to it. Dealing about 50 potency higher than Despair for each Astral Soul. Meaning a total of 490 Potency at max power. If you somehow don't have any Astral Hearts, It would just remain as Despair.

If they're really intent on it being a new button, then it could still have the effect of using up whatever Astral Souls you have.