Myriad is a triggered ability that triggers when a creature with it attacks. For each opponent other than the player you're attacking (the defending player), you may create a token that's a copy of that creature. Those tokens enter tapped and attacking that opponent or a planeswalker they control. At end of combat, all tokens created this way are exiled. If a card has "myriad, myriad," the process happens twice, creating two tokens for each other opponent.
In play, myriad shines in multiplayer games, letting you pressure every opponent at once without spreading your actual creatures thin. The token copies can trigger attack abilities or deal combat damage, and because they're exiled at end of combat, they don't stick around to be blocked or removed later. It's a great way to get extra value from one attacker.
