The wizard is mentioned by Adomnán as being close to the king and that he also possessed an Irish slave-girl who had possibly been a war captive.
Columba then went down to the river and told his companions that an angel at that moment had struck Broichan, and his cup had fallen from his hand and he was deathly ill.
Moments later two messengers from King Bridei came to inform Columba of these events as they happened, and Columba gave his own messengers a pebble from the River Ness and told them that if Broichan agreed to release the slave-girl, then they were instructed to place the pebble in a cup of water and make him drink it, but if Broichan refused, then he would die immediately.
They then went back to the fortress and Broichan agreed to release the girl, and so they placed the pebble in a cup of water, and it miraculously floated on top.
Columba then called on Christ's name and ordered the sails to be raised, and miraculously the boat then went off at a high speed directly into the oncoming wind that was blowing against them.