"CountDown" house rules for multiplayer EDH
The monday "CountDown" magic player community changed magic the gathering rules because we often play 2HG and 3HG. The goal is to make this game and some unbalanced mechanics fairer.
1. Disadvantage "each opponent" effects
1.1. Spells and abilities
Spells, activated or triggered abilities that:
- effect only multiple opponents (e.g. Agent of Masks, Wheel and Deal)
- AND if any are only able to target permanents or abilities (e.g. Planeswalker or Creatures) (e.g. Decoy Gambit) an opponent controls
- AND have no additional cost to choose multiple targets (counterexamples Call the Coppercoats, Fireball)
OR
- are seperating the players into multiple categories, like friend and foe (e.g. Pir's Whim)
will only effect one opponent.
The controller chooses to pay {1} additional generic mana while casting the spell or announcing the ability for any other opponent after the first one to effect all of them. The controller decides which one is the initially effected opponent, if not paying the additional cost.
1.1.1. Example:
In a 3HG game, a player plays Windgrace's Judgment. Then he can decide to destroy one permanent of an opponent or to pay two additional generic mana to destroy 3 nonland permanents.
1.2. State-Based effects
When a state-based actions that only effects opponents (e.g. God-Pharaoh's Statue) begins to take effect, the source controller chooses state-based one opponent that will be effected. When the source of the state-based action changes the controller, the new controller chooses one of his opponents as state-based action (see Example 1.2.2.). The decision who will be affected is not a copyable property of the card (see Example 1.2.3.,1.2.4.).
1.2.1. Example:
The effect of Grand Arbiter Augustin IV "Spells your opponents cast cost {1} more to cast." will only effect one opponent.
1.2.2. Example:
A Grand Arbiter Augustin IV is on the battlefield controlled by Player A. As opponent is Player B choosen. Now Player B plays a Sower of Temptation and takes control of Grand Arbiter Augustin IV. Than Player B chooses a new Player affected by Grand Arbiter Augustin IV static ability.
1.2.3. Example:
A Clone is played and copies a Anafenza, the Foremost. The controller of the Clone chooses a new opponent for the state-based affect of Anafenza, the Foremost.
1.2.4. Example:
A Nanogene Conversion is played targeting a Anafenza, the Foremost. Than for each creature on the battlefield that becomes a copy of Anafenza, the Foremost the controller chooses an opponent for the state-based ability.
1.3. Opponent individually triggered abilities
All triggered abilities that trigger separately whenever an opponent or a permanent that opponent controls fullfills a condition the rule State-based effects applies.
1.3.1. Example:
The trigger of Smothering Tithe will only trigger for the chosen opponent.
1.3.2. Example:
During resolution of Punishing Fire the owner of the spell chooses an opponent, who has to gain life in order to trigger its graveyard effect.
2. Phase and turn manipulation
All effects that manipulate phases or turns of only one team, such as extra turns, apply only to the player controlling the effect or to the target of the effect, not to the entire team.
2.1. Example:
Activating the card Mindslaver will only control the turn of one player. The player will be chosen by the ability controller.
2.2. Example:
Playing the card Time Stop will still end the turn for all players of the active team.
3. Determine life points of a player
Life points of a player from a team are equal to the current life points of the team divided by the number of players in the team, rounded up.
3.1. Example:
In a 2HG game, a player of a team that has 17 life points becomes the target of a Beacon of Immortality that reads, among other things, "Double target player's life total." This player gains 9 life points, so that the team ends up with 26 life points.
3.2. Example:
In a 2HG game, one player controls Test of Endurance, an enchantment that reads "At the beginning of your upkeep, if you have 50 or more life, you win the game." If his team's health is 100 or more, his team wins the game.
3.3. Example:
In a 2HG game, a player of a team that has 25 life points becomes the target of an ability that reads: "The life score of target player becomes 10." That player's life score is considered to be 13, so that the player loses 3 life points. The team ends up with 22 life points.
4. Put Commander into library
If an effect is putting a Commander card into its owner's library, the owner has NOT the option to put it into the Commander Zone.
4.1. Example:
Terminus is played. If a Commander card is on the battlefield, it's put on the bottom of the library as well.
4.2. Example:
Chaos Warp targets a Commander. The card is shuffled into the library.
5. Multiplayer poison rule
The amount of poison counters a team requires to lose the game is defined by the formula 5 + 5 * x. Where x is the number of players in one team.
5.1. Example:
In a 3HG match one team needs 20 poison counters to lose the game.
Glossar
Expaining some wordings for clarification.
2HG
Two-Headed Giant playmode or 2 vs. 2
3HG
Three-Headed Giant playmode or 3 vs. 3
Command Zone
defined in Play rule 6 in the official Commander rules.
Generic Mana
A generic mana cost is a mana cost that can be paid with mana of any type; meaning mana of any color, as well as colorless mana.
Symbol: 
From the glossary of the Comprehensive Rules (September 1, 2023—Wilds of Eldraine)
Generic Mana
Mana in a cost represented by numerical symbols (such as {1}) or variable symbols (such as {X}) that can be paid with mana of any type. See rule 107.4.
State-Based Actions
Game actions that happen automatically whenever certain conditions are met. See rule 704, “State-Based Actions.”