Every oath holds power. In a land where the Earth spawns terrible beasts capable of moving through all but stone, the Sky bestows power to those bound by Oaths. As earthbound creatures invade, bringing with them chaos and whispers of a coming war, protector and Overseer Tehran fights to keep his people safe. After losing his best friend to the beasts and with the stability of the region deteriorating, Tehran heads to the Capital to plead for support from the Order of the Sky. When his friend's widow stows aboard the transport, determined to hold the Order accountable for the death of her husband, Tehran's plans are derailed. Torn between upholding his responsibility as an Overseer or honouring his friend's memory, Tehran must either choose the Oaths that have forged him or trust new bonds to guard the fate of the Mercurial.