He was accused of bewitching townsfolk, preaching witchcraft, and, along with Agnes Sampson and others, raising storms to sink the fleet of King James VI of Scotland and his wife Anne of Denmark as they returned from Copenhagen, having been married in Oslo.

To verify this, Fian caused the same gentleman to come before the presence of King James in the king's chamber on 24 December 1590, where he purportedly bewitched the man, causing him to be in a hysterical fit for an entire hour of screaming, contorting and jumping high enough to touch the ceiling of the chamber; after the hour ended, the gentleman declared no memory of the event, as if he were asleep.

The next morning, he confessed that during the previous night, the Devil came to him in his cell dressed in all black with a white wand, demanding Fian to continue his faithful service, according to the first oath and promise of their agreement.

[3] He endured the torture of having his fingernails forcibly extracted, then having iron pins thrust therein, the pilliwinks, and the boot to crush his feet until they were so small that they were no longer usable.

The English ambassador Robert Bowes recorded that during his execution Fian denied his confession, saying he told those tales by fear of torture and to save his life.