Mythra summons Rex, who says "Mythra, now!" and performs an underhand slash to transition into a cutscene. In the cutscene, Mythra says "Concentrate, Rex.", and she and Rex hold the Aegis Sword together and point it into the sky to summon a beam of light, shouting the move's name. Transitioning into normal gameplay, they point the Aegis Sword forward to rain smaller beams of light from the sky down onto the opponents, whilst Rex says "And, strike!", before they fires a light beam from the sword to finish the move.
In Xenoblade Chronicles 2, Sacred Arrow is Mythra's Level 4 Blade Special. The move's animation in Smash is a recreation of its animation in battles in Xenoblade Chronicles 2.