Skull keys

Skull keys are a more advanced form of lockpicks. While the common lockpick can only open normal locks, skull keys can also open magically locked crates and doors. They are therefore extremely useful when such a lock has to be opened and the area doesn't allow the casting of the spell Unlock Magic.

Unfortunately, a skull key is only good for a single lock. Nonetheless, it is a must-have for every adventurer, because with it even non-magic users can open magic locks. It appears in  Ultima V and  Ultima VII.

Age of Enlightenment
During the tyrannical regime of the once noble regent Lord Blackthorn, a brilliant smith called Shenstone invented the skull keys, an important tool which can open doors who are magically locked.

The only other appearance of a skull key was early in the Age of Armageddon.

Spoiler
Skull keys were very important for entering Stonegate, and for retrieving Lord British's Crown from the Palace of Blackthorn, since in these areas, magic could not be used.

Ultima V
Skull Keys may be found at the following locations in Ultima V:
 * A tree stump frequented by Shenstone in Minoc housed new skull keys daily.
 * From Kristi at Serpent's Hold for 100 gp.

Ultima VII
Skull Keys may be found at the following locations in Ultima VII:
 * Found on one of the Valarian Islands west of Trinsic.

Trivia

 * The skull keys in Ultima VII, replaced the lockpicks, and never broke.
 * It is possible to get more than 5 keys from Kristi. So long as you never give her your name, you can keep asking her for keys.