Raiyuu, Storm's Edge

First strike
Whenever a Samurai or Warrior you control attacks alone, untap it. If it's the first combat phase of the turn, there is an additional combat phase after this phase.

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A creature attacks alone if it's the only creature declared as an attacker during the declare attackers step. For example, Raiyuu's triggered ability won't trigger if you attack with multiple creatures and all but one of them are removed from combat.

Notably, the triggered ability doesn't give you any additional main phases. This means that you will move directly from the end of combat step of one combat phase to the beginning of combat step of the next one.