Erik the Viking/WMG

The Cloak Invisible is a Cloak of Invisibility To Virgins.
The two people on whom it worked, of a group of vikings and a group of hedonistic people, were the Christian missionary and Aud's "father". Most sects of Christianity prefer that their clergy not have sex, and usually make it a requirement, so that covers Harald. As for King Arnulf, he is a very naive and gullible married man whose wife had a cloak of invisibility that worked only on him, and if the guess is correct, it would cease to work on him if he ever lost his virginity. Think about it.