Wondrous item, uncommon
This brass door knocker is fashioned after a lion's head. You can attach the knocker to an unlocked door by firmly pressing it against the door for 1 minute. The door then becomes locked, as if by the arcane lock spell. You choose the door's password when attaching the knocker to it. Speaking the correct password to enter the door causes the knocker to emit a rumbling purr. If the door doesn't have a handle, a brass handle in the design of a lion's paw appears where one would normally go. The handle also has a physical lock, although there isn't a key made to fit it.
If a creature attempts to pick the door's lock or break the door down, the knocker transforms into a Large lion, including any handle it may have created. The lion appears in the nearest unoccupied space before the door and attacks the creature and its allies. Unless the creatures know the nature of the door knocker, they are almost always surprised. The transformed lion is made of brass: it uses the lion's statblock, except that it's a construct instead of a beast, it has immunity to poison damage and the poisoned condition, and it has resistance to bludgeoning, piercing, and slashing damage from nonmagical attacks.
The lion returns to its normal form and reappears on the door after 1 minute. If the lion is reduced to 0 hit points, it returns to its normal form on the ground at the foot of the door. Regardless, the lion regains all its hit points when it returns to its normal form.
"Okay, the password is 'meow'."