This is an ordinary but large normal key, with a faint magical aura.
When touched to any lock, it may (50%) open it. (Note that this
may trigger unremoved traps.) If it does not function, it cannot
affect that lock. The key may also be used to negate a Wizard Lock
or Hold Portal spell effect (50% chance of success) if a command
word is spoken while the user touches the locked portal.

An unusual side effect may also occur. With each touch, the key
may create a skeleton on the opposite side of the portal affected.
The chance of this occurrence is 3%, cumulative with each use;
thus, the DM and player should keep track of the number of times
the key has been used. The skeleton thus created cannot be Turned
by a cleric; it is a magical construct, not a true skeleton. In addition,
the skeleton will be that of the race most commonly found
within 120' of the key at the time of use; for example, in a bugbear
lair, a bugbear skeleton will appear. (See DMG pg. 41 for guidelines on monster skeletons, and MM2 pg. 109 for animal types.)

When first found, the key has a 0% chance of creating a skeleton.
If sold, the item retains its current percentage. When the
chance reaches 99%, the next use causes the key to Teleport to a
random location.