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, these are a must have for every adventurer, because with them even non-magic users can open magic locks. It appears in Ultima V and Ultima VII.

History
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.

The only other appearance of a skull key was early in the Age of Armageddon. Found on one of the Valarian Islands west of Trinsic, it replaced lockpicks, and never broke.

Trivia
They were available in Ultima V from only two locations:
 * A tree stump frequented by Shenstone in Minoc housed new skull keys daily
 * From Kristi at Serpent's Hold. 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.