Danna-sama no Atama ga Kemono na no wa Doumo Watashi no Sei Rashii

A princess named Rosemarie experiences a peculiar phenomenon: when others display negative emotions, their heads transform into those of monstrous beasts. This makes court life difficult, as she cannot trust anyone. She finds an exception in Prince Claudio, whose appearance remains unchanged, and they marry. Later, she discovers that Claudio's human form is not due to kindness, but because she once unknowingly took his mana. Now, he is turning into a beast and needs his mana back to survive. Rosemarie is determined to find a way to restore it, even as time runs short.