Astral Arena

No more than one creature can attack each combat.
No more than one creature can block each combat.
Whenever chaos ensues, Astral Arena deals 2 damage to each creature.

Planechase Anthology Planes

No more than one creature can attack each combat.
No more than one creature can block each combat.
Whenever you roll {Chaos}, Astral Arena deals 2 damage to each creature.

Planechase 2012 Planes

No more than one creature can attack each combat.
No more than one creature can block each combat.
Whenever you roll chaos, Astral Arena deals 2 damage to each creature.

Astral Arena’s abilities stop more than one creature from being declared as an attacker or as a blocker each combat. However, it doesn’t affect creatures that are put onto the battlefield attacking or blocking.

If a turn has multiple combat phases, either a different creature or the same one can attack or block in each of them.