Mausoleum. It is not available in the Market and can only be pieced together by winning Rusty Fragments via Invasions. In every Invasion there is 1/5 Chance you'll get a piece (Seen in the Invasion Files of Zombie Farm).
After you combine the fragments, you can "Use" the Rusty Key to unlock the Mausoleum and use it to store zombies.
Text: A spooky key!
Other notes: Listed as a Boost Item