A Storm Cloak is a magical item capable of negating magic in Ultima VI.
While wearing it, all magic in the area is negated, essentially crippling magic-strong foes like dragons and daemons. However, if this wondrous cloak is used too often, its negation ability will run out, and the cloak will vanish.
Only three Storm Cloaks were found. The first one was given to the Avatar by Jaana in Yew. She kept it in her home, and gladly gave it to the Avatar after joining the party. The second Storm Cloak was in the treasure of Hawkins, and Homer demanded it as payment for giving the Avatar detailed instructions on Captain Hawkin's buried treasure. The final cloak could be found on the body of an unfortunate adventurer in the Swamp Cave.