Place Sticky Pistons behind this wall connected to a trigger.
Behind the pistons, place a contained mob (or a spawner).
When the trigger is pulled, the wall retracts, revealing the threat.
Extension Phase (0.3 seconds): The piston head moves. At the exact midpoint, a 1x1x1 invisible portal opens.
Retraction Phase (0.2 seconds): The portal collapses. If a mob entity is pathfinding through that exact coordinate at the moment of collapse, it is torn from the Thin World into your reality.
