In 1372 DR, Witchthorn had a relationship with the Adarbrent noble family of Waterdeep. For decades the family sent its firstborn child of each generation to pledge an oath to Witchthorn. The children performed an unknown service to the king by their eleventh year and in exchange the family businesses and their homes were protected by secret fey guardians. This arrangement was unknown to all but the eldest son and daughter of each generation.
In 1489 DR, the arrival of Claugiyliamatar corrupted Ishaldra and her treant bodyguard. Thus Witchthorn, who had become king, unleashed his evil against the desecrater of the forest, but Old Gnawbones was too powerful for him.
Ultimately, an adventuring party hired to defeat Old Gnawbones gained an audience with the fey king. Witchthorn agreed to give his help in order to defeat the green dragon, but only if one of the adventurers made an oath with him. Presumably, the adventurers agreed to the pact.