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Did you ever play that game as a kid where groups of 2 people would stand arm-in-arm at random locations in an area, then there were two kids in the middle (one chasing the other) and whenever the one being chased latched onto one of the groups of 2, the person they weren't touching in that group became the one being chased? This game went by a variety of names, but where I learned it, we called it Fox and Rabbit, with the groups of 2 representing rabbit-holes. This format is based on that game.

The Rules
Fox-and-Rabbit Magic is a one-vs-many format in which the "Fox" wins by eliminating the "Rabbit," and the Rabbit and his/her team wins by eliminating the Fox. It can be played by any number of players as long as the Rabbit's team is an odd number, and the Fox has special starting life-total and starting/maximum hand size rules. I recommend that the Fox use a large deck, such as an EDH deck with its general shuffled into it because of how large his/her starting hand is, making him/her especially vulnerable to milling.

Life totals and Staring hand sizes
The Fox's starting life total is ten times the number of his/her opponents, and the Fox's starting/maximum hand size is 7 plus the number of his/her opponents. The Rabbit's team starts at normal life/hand rules. At the beginning of the game, the Rabbit's team confers and selects who will be the first Rabbit, as well as who will be paired in what order to form rabbit-holes.

Teammates
The Fox cannot be attacked or targeted except by the Rabbit or him/herself and cannot attack anyone or target anyone but the Rabbit. All players on the Rabbit's team are considered teammates, and therefore cannot attack each other.

"Latching onto" your teammates
If the Rabbit would be attacked by the Fox, he/she can instead latch onto any rabbit-hole on any side. He/she is no longer considered the Rabbit, and is part of that rabbit-hole. The person on the "other side" of that rabbit-hole is now the Rabbit and loses 3 life, as well as becomes the new target for all creatures the Fox was attacking with and all spells on the stack that were targeting the Rabbit.

Victory conditions
If a member of a rabbit-hole loses the game, the other member of that rabbit-hole loses the game. If the Fox loses the game, the remaining players win the game. If the Rabbit loses the game, the Fox wins the game and all other players lose the game.

Notes
This format is still in its infant stage and most likely needs some fine-tuning. The biggest problem it seems is that I can't think of what the best starting life total for the Fox would be. Too high, and he's nigh unkillable. Too low, and he's just an easy target. Feedback from anyone who tries this would be appreciated. Also I don't know if the people forming rabbit-holes should be allowed to target/attack the Fox or not. As I wrote it, the can, but I'm not sure if that makes it too hard on the Rabbit. Again, playtester feedback would be appreciated.