Myriad

Myriad is a triggered keyword ability introduced in Commander 2015. By way of token copies, it enables a creature to be used to attack multiple players at once.

Description
Whenever a creature with myriad attacks, its controller may create token copies of that creature for each other opponent, which are exiled at the end of combat. Those tokens may also attack planeswalkers controlled by the corresponding player.

Rulings

 * The term "defending player" in the myriad rules (or any other ability of an attacking creature) refers to the player the creature with myriad was attacking at the time it became an attacking creature this combat, or the controller of the planeswalker the creature was attacking at the time it became an attacking creature this combat.
 * If the defending player is your only opponent, no tokens are put onto the battlefield.
 * You choose whether each token is attacking the player or a planeswalker they control as the token is created.
 * Although the tokens enter the battlefield attacking, they were never declared as attackers. Abilities that trigger whenever a creature attacks won't trigger, including the myriad ability of the tokens. If there are any costs to have a creature attack, those costs won't apply to the tokens.
 * The token creatures all enter the battlefield at the same time.
 * Each token copies exactly what was printed on the original creature and nothing else. It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.
 * Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any "as [this permanent] enters the battlefield" or "[this permanent] enters the battlefield with" abilities of the copied creature will also work.
 * If myriad creates more than one token for any given player (due to an effect such as the one Doubling Season creates), you may choose separately for each token whether it's attacking the player or a planeswalker they control.