Sokenzan is a common Plane — Kamigawa card from Planechase Anthology Planes (OPCA), collector number 72. Current market price: $2.26, TCGplayer market prices via Scryfall.
All creatures get +1/+1 and have haste. Whenever chaos ensues, untap all creatures that attacked this turn. If it's a main phase, there is an additional combat phase after this phase, followed by an additional main phase.