geas

A (generally magical) vow, obligation or injunction placed upon someone to do or not do something, which typically brings harm if violated and blessings if obeyed.

It can send us where it will with a task laid upon us—a geas, if you like.

Galaad stood up again and turned to Mrs. Whitaker. 'Gracious lady, keeper of the Holy of Holies, let me now depart this place with the Blessed Chalice, that my journeyings may be ended and my geas fulfilled.'

A mystical compulsion.

The memory came upon him like a geas, overwhelming his revulsion, numbing his heart.

A geas is your own personal haunting by yourself! […] Kassandra, a Greek prophetess who always envisioned dreadful happenings, had a geas on her. She might have wanted to be a queen or a housewife, a warrior or a merchant, but she wasn't (even though a geas won't interfere in any of your pursuits). She became legendary for the geas that propelled her to prophesy dreadful happenings.