The arena for this is a 12 square x 12 square platform suspended in the Elemental Chaos.
Any creatures on a tile when it collapses must immediately make a saving throw. On a successful save, they take 5 fire, force, thunder, and lightning damage, shift 1 square to an adjacent tile if able, and fall prone. On a failed save or failed movement, they die instantly. This leaves a permanent hole in the arena.
At the end of a player’s turn, that player immediately collapses 2 outer tiles.
An outer tile is a square that has at least 1 side adjacent to the unbounded open area surrounding the platform.
A creature rolling a natural 1 on an attack roll falls prone after the attack. The tile they occupy then collapses.
If a creature scores a critical hit, along with dealing maximum damage, they must also collapse 1 unoccupied adjacent tile.
Once per turn, a creature moving or jumping over a hole, including diagonally over a corner, deals an extra 5 fire, force, thunder, and lightning damage on their next melee attack this turn.
Trigger: You reduce 2 or more non-minion enemies with at least 1 hit point to 0 hit points or fewer during the same turn.
Effect (Free Action): Gain an action point. You may spend more than one action point per encounter, but only one action point per round. You may also choose to collapse 1 adjacent tile.
Trigger: You start your turn dead or begin your first turn of the encounter.
Effect (No Action): You regain all hit points and healing surges, remove all failed death saves, conditions that don't last through the encounter, refresh all encounter powers and power points, and teleport to an unoccupied outer tile of your choice.
1: The inclusion or not of the word “unoccupied” in these effects is deliberate.
2: As per 4E Rules As Written, prone creatures cannot shift.