Lord British's Crown

Lord British's Crown is one of Lord British's Royal Crown Jewels. It has a powerful magic effect: whoever wears it negates the use of magic by their foes. Even very powerful creatures like the Shadowlords cannot withstand its powers. It appears in  Ultima V.

Description
The crown was the first item seized by Lord Blackthorn after being corrupted by the Shadowlords. He hid it on top of the Palace of Blackthorn behind a magically locked door, guarded by animated stone gargoyles. Landon and Sir Simon told the Avatar of its importance. The Avatar managed to regain the crown, using it to withstand demonic powers in dungeon Doom. After that, the crown stayed with Lord British.

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.

Trivia

 * A little gag in the series is the fact that in no two Ultimas does the crown looks the same.
 * The Crown acts as an anti-magic field when it's worn, but only affects non-party members.
 * In Ultima V the Crown has to be "used" to wear it, and it doesn't replace any pieces of your character's armor. While it is worn, all magic cast by anyone near the party, other than the party members themselves, is completely negated. 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).