Progenitor God Ruptured Heaven
Byleth begins by lashing forward with his/her sword, shouting "I'll finish with this!" (これで終わります！, Here's the end!). The game then shows Sothis in the background giving Byleth power, turning his/her hair green, and Byleth readies his/her sword, before extending it into its whip form, and lashing out once more with extreme force.
This is an upgraded Smash version of the existing Ruptured Heaven combat art from Fire Emblem: Three Houses. Ruptured Heaven is exclusive to Byleth while wielding the Sword of the Creator. It adds 30% of the user's magic to the weapon's might and deals effective damage to dragon units. In Three Houses, Ruptured Heaven does have an upgraded version known as Sublime Heaven, attained as part of the upgraded Sublime Creator Sword when Byleth and Sothis fuse in Chapter 10. Byleth's green hair as a result of fusing with Sothis ironically makes it impossible for this attack to be Ruptured Heaven, as it is replaced with Sublime Heaven from that point onward.
Before Fire Emblem: Three Houses, it had appeared in Fire Emblem Fates and Fire Emblem Heroes under the names Rend Heaven and Ruptured Sky respectively. In the former game, it is associated with the Basara class, having the ability to add half of the enemy's strength or magic to the user's own strength or magic (depending on whichever stat the skill's user is currently using to attack the enemy). Meanwhile, in the latter, it causes the user's damage output to be boosted by 20% of the target's Attack, (further boosting it to 40% if the target is a non-human foe). This form of the skill is considered to be an upgraded form of Draconic Aura.