Ok here is the situation, I'm playing with some friends in a 1v1v1v1 ( yah I know...) I play Breaker of Armies and as you know it's ability "All creatures able to block this creature do so" and my friends says if I attack opponent A it also forces opponent B and C to block at the same time. I really doubt this is legal because you cannot block for an opponent, but they want a second opinion.
An attacking creature can only be blocked by the player who is being attacked by it, or who controls the planeswalker it attacks.
509.1a The defending player chooses which creatures that he or she controls, if any, will block. The
chosen creatures must be untapped. For each of the chosen creatures, the defending player
chooses one creature for it to block that’s attacking him, her, or a planeswalker he or she
controls.
EDIT:
Of course, in a Casual game, the players can agree on a houserule to allow blocking for other players (usually teammates), but without such a rule, it's everybody for him/herself.
I assume your multiplayer game uses the attack multiple players option. In that case, a creature can block Breaker of Armies only if Breaker of Armies is attacking that creature's controller or a planeswalker he or she controls (C.R. 802.4a). Because of this, Breaker of Armies's requirement won't apply to opponent B or C if Breaker of Armies attacks opponent A.
Ok here is the situation, I'm playing with some friends in a 1v1v1v1 ( yah I know...) I play Breaker of Armies and as you know it's ability "All creatures able to block this creature do so" and my friends says if I attack opponent A it also forces opponent B and C to block at the same time. I really doubt this is legal because you cannot block for an opponent, but they want a second opinion.
The rules for a multiplayer game should be agreed upon by all players before the game begins. There are many variant rules and options that are possible for a game.
But unless none of the variants and options listed in the official rules allow you to block creatures attacking a player who is not your teammate.
Your game here would be Free-For-All game with the Attack Multiple Players combat option.
806. Free-for-All Variant
806.1. In Free-for-All multiplayer games, a group of players compete as individuals against each other.
806.2. Any multiplayer options used are determined before play begins. The Free-for-All variant uses the following default options.
806.2b. Exactly one of the attack left, attack right, and attack multiple players options must be used. See rule 803, "Attack Left and Attack Right Options," and rule 802, "Attack Multiple Players Option."
802. Attack Multiple Players Option
(...)
802.4a. A defending player can block only with creatures he or she controls. Those creatures can block only creatures attacking that player or a planeswalker that player controls.
So, B's and C's creatures aren't able to block your Breaker of Armies that is attacking opponent A.
509.1a The defending player chooses which creatures that he or she controls, if any, will block. The
chosen creatures must be untapped. For each of the chosen creatures, the defending player
chooses one creature for it to block that’s attacking him, her, or a planeswalker he or she
controls.
EDIT:
Of course, in a Casual game, the players can agree on a houserule to allow blocking for other players (usually teammates), but without such a rule, it's everybody for him/herself.
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)
The rules for a multiplayer game should be agreed upon by all players before the game begins. There are many variant rules and options that are possible for a game.
But unless none of the variants and options listed in the official rules allow you to block creatures attacking a player who is not your teammate.
Your game here would be Free-For-All game with the Attack Multiple Players combat option.
806. Free-for-All Variant
806.1. In Free-for-All multiplayer games, a group of players compete as individuals against each other.
806.2. Any multiplayer options used are determined before play begins. The Free-for-All variant uses the following default options.
806.2b. Exactly one of the attack left, attack right, and attack multiple players options must be used. See rule 803, "Attack Left and Attack Right Options," and rule 802, "Attack Multiple Players Option."
802. Attack Multiple Players Option
(...)
802.4a. A defending player can block only with creatures he or she controls. Those creatures can block only creatures attacking that player or a planeswalker that player controls.
So, B's and C's creatures aren't able to block your Breaker of Armies that is attacking opponent A.