You'll have no choice but to put Arianna out of her misery, killing her along with the child. Doing so drops Arianna's Umbilical Cord, which reads: “Every Great One loses its child, and then yearns for a surrogate, and Oedon, the formless Great One, is no different.

When should I kill Arianna?

Once you have defeated Micolash, Host of the Nightmare, she'll move to the aqueduct between the Tomb of Odeon and Cathedral Ward lamps. You must kill both Arianna's newborn and Arianna herself in order to acquire One Third of Umbilical Cord and Arianna's Shoes.

Who killed Arianna?

Ariana died when she was accidentally struck by a curse in a three-way duel between her older brothers and Gellert Grindelwald, the soon-to-be infamous Dark Wizard revolutionary. This event would have a profound impact on both her brothers' lives.