Source: Phandelver and Below - The Shattered Obelisk

Wondrous Item, Very Rare

You can use an action to place this 1-inch-diameter granite sphere on the ground and speak its command word, which is “petrification” in Terran. The sphere rapidly grows into a stout tower that remains until you use an action to touch the tower and speak the command word again, whereupon the tower shrinks back to a 1-inch-diameter granite sphere. The tower must be empty to shrink in this way. The tower bristles with muddy knobs that constantly extrude and retract across its surface, as though the tower were breathing through a coating of thick mud.

Each creature in the area where the tower appears must make a DC 15 Dexterity saving throw, taking 10d10 bludgeoning damage on a failed save, or half as much damage on a successful one. In either case, the creature is pushed to an unoccupied space outside but next to the tower. Objects in the area that aren’t being worn or carried take this damage and are pushed automatically.

Whenever it expands, the mudslick tower merges with any natural stone it touches, awkwardly tipping and wedging itself to touch as much natural stone as it can.

The tower is 20 feet on a side and 30 feet high, with arrow slits on all sides and a battlement atop it. Its interior is divided into two floors, with a ladder running along one wall to connect them. The ladder ends at a trapdoor leading to the roof. When activated, the tower has a small door on the side facing you. The door opens only at your command, which you can speak as a bonus action. It is immune to the Knock spell and similar magic, such as that of a Chime of Opening.

Although it looks like stone, the tower is made of adamantine, and its magic prevents creatures from tipping it over. The roof, the door, and the walls each have 100 hit points, immunity to damage from nonmagical weapons excluding siege weapons, and resistance to all other damage. While merged with natural stone, the mudslick tower has immunity to all damage. Only a Wish spell can repair the tower (this use of the spell counts as replicating a spell of 8th level or lower). Each casting of Wish causes the roof, the door, or one wall to regain 50 hit points.