Lord British's Crown

Lord British's Crown is one of Lord British's Royal Crown Jewels. It has a powerful magic effect: while it is worn, all magic cast by anyone near the party, other than the party members themselves, is completely negated.

In Ultima V, the Crown had been somehow obtained by Lord Blackthorn, who used it to prevent magic from being used in his castle. He kept it in the top floor of his castle. Landon and Sir Simon told the Avatar of its importance. The Avatar recovered the Crown and used it when rescuing Lord British. After this, the Crown was restored to its rightful owner.

In Ultima V the Crown has to be "used" to wear it, and it doesn't replace any pieces of your character's armor. The Crown was treated as a 'global effect' when in use, and in Ultima V only one such effect could be in use at a time. Thus casting Time Stop, for instance, would remove the Crown's effects--it would have to be "used" again (preferrably once Time Stop ran out, or it would end up cancelling the spell before it expired naturally).

Ultima V: Lazarus
In Ultima V: Lazarus the Crown has to be worn by one of the party members as a head piece of armor. It provides 2 defense points and negates all magic cast by anyone but the wearer. The Shadowlords are not affected by the Crown's negation effect.