Helm of Chatzuk 
Text:
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: Target creature gains banding until end of turn. (Any creatures with banding, and up to one without, can attack in a band. Bands are blocked as a group. If any creatures with banding a player controls are blocking or being blocked by a creature, that player divides that creature's combat damage, not its controller, among any of the creatures it's being blocked by or is blocking.)
Rulings:
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10/1/2008
If a creature with banding attacks, it can team up with any number of other attacking creatures with banding (and up to one nonbanding creature) and attack as a unit called a “band.” The band can be blocked by any creature that could block a single creature in the band. Blocking any creature in a band blocks the entire band. If a creature with banding is blocked, the attacking player chooses how the blockers’ damage is assigned.
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10/1/2008
A maximum of one nonbanding creature can join an attacking band no matter how many creatures with banding are in it.
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10/1/2008
Creatures in the same band must all attack the same player or planeswalker.
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10/1/2009
If a creature in combat has banding, its controller assigns damage for creatures blocking or blocked by it. That player can ignore the damage assignment order when making this assignment.
Legality:
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standard: Not legal
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future: Not legal
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frontier: Not legal
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modern: Not legal
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legacy: Legal
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pauper: Not legal
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vintage: Legal
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penny: Not legal
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commander: Legal
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1v1: Not legal
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duel: Legal
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brawl: Not legal
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oldschool: Legal
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historic: Not legal
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pioneer: Not legal
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gladiator: Not legal
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premodern: Legal
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historicbrawl: Not legal
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paupercommander: Not legal
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alchemy: Not legal
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explorer: Not legal
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predh: Legal
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oathbreaker: Legal