Here is another installment of exTRAPaganza, our Thursday over-the-top trap series.
A large stone is by the door. It contains a keyhole.
A special key is required to open the door. It is solid stone and will not open otherwise.
The lock can be picked, but it is hard. It requires a hard dispel magic and 3 hard Dexterity checks with thieves’ tools with no more than 2 failures. If the dispel magic fails, or the lock picking fails, it causes the trap to go off.
When the trap goes off, the entire area extending out 100′ from the building perimeter turns into quicksand and the building begins to sink. All targets in this area have their speed reduced to 1/4. Each round, they must make a hard Strength (Athletics) check or sink under the surface, where they start drowning and are considered grappled. On a success, they can move and the grappled condition is removed.
Once the building sinks, which taked 1d8 + 8 rounds, the ground becomes solid again, instantly killing any creature grappled by it.