Historical Rules · 26 August 2016
Rule 702.115a
Official source text from the 26 August 2016 Rules release
Myriad is a triggered ability that may also create a delayed triggered ability. “Myriad” means “Whenever this creature attacks, for each opponent other than defending player, you may put a token that’s a copy of this creature onto the battlefield tapped and attacking that player or a planeswalker he or she controls. If you put one or more tokens onto the battlefield this way, exile the tokens at end of combat.”