Planechase is a multiplayer format for Magic: The Gathering first release in 2009. I've only been lucky enough to play it once or twice, and at first I thought we were playing it wrong, but it appears we just played a slight variant.
Players either create a custom deck of 60 cards or use the specifically designed pre-constructed deck available in the Planechase product line. Each player then needs a 10 card Planar Deck consisting of "Plane" cards — oversized cards representing the different planes in the Magic multiverse that are only used in this format — and/or "Phenomenon" cards (added in the 2012 Edition). All of the cards in each individual Planar Deck must be unique and not share the same name, including the Phenomenon cards. Furthermore, if Phenomenon cards are used, they are restricted to 2 in each deck.
A "Planar Die" is also used, which is provided with the various Planechase product.
It is possible to use one universal Planar Deck for all players (which is how I played), consisting of the same requirements/restrictions — 10 Plane and/or Phenomenon cards (I'm assuming up to 2 Phenomenon cards per player) for each player.
Plane cards comprise of one always active ability that either affects all players or whoever is taking their turn and one "Chaos" ability, which is a triggered ability of sorts. Essentially, Plane cards modify the gameplay. As do Phenomenon cards.
At the beginning of the starting player's turn, the first Plane cards is flipped from their (or shared) Planar Deck, representing which plane the players are currently in.
As mentioned, these oversized cards modify play throughout the game. On a player's turn, anytime they may cast a Sorcery, they may roll the Planar Die (having 4 blank sides, 1 Planeswalker symbol dubbed "Planeswalk", and 1 Chaos symbol). Nothing happens on a blank roll (unless rules state differently); a Planeswalk roll signifies that the players are shifting planes and the current active Plane card is put on the bottom of its owner's Planar Deck (or the shared deck) and they then reveal the top of their Planar Deck (or, again, the shared deck), which becomes the new active plane; and a Chaos roll triggers the ability of the active Plane card.
A player may roll the Planar Die as many times as they want at the cost of one colorless mana for each time the die has already been rolled that turn.
If a player reveals a Phenomenon card, they follow the instructions on the card and then immediately Planeswalks away (revealing the next card of the Planar Deck).
Unfortunately, I missed out on all of the Planechase releases, and while I think the Anthology product is still available, it's pretty pricey.
From what I remember, though, I really enjoyed the games I played and would love to play more of this multiplayer variant!
Do you currently play Magic: The Gathering? Are you a fan of Planechase? If so, what's your favorite new card and/or deck from the series? Did you like the addition of Phenomenon cards? Do you think Planechase should be further supported with more products? What would you add to or change with the format?