Just think about it, for a second.

The Sword of the Creator requires a Crest of Flames, and a crest stone, which Byleth has in canon, so that checks out.

But SSBU Byleth can also use Aymr, so they must also have the crest of Seiros, and with two Crests comes white hair and nightmares. But it doesn’t stop there, because Areadbhar, which SSBU Byleth can ALSO use, requires the Crest of Blaiddyd. I think you can see where this is going. No one in the Three Houses universe has three Crests - that would probably melt any normal human into demonic sludge from the inside, but maybe Byleth would survive because of Sothis… MAYBE. However, SSBU Byleth goes EVEN FURTHER BEYOND that, because they can also use Failnaught, which requires ANOTHER Crest: the Crest of Riegan. That’s FOUR crests. FOUR. Sothis must be working overtime to prevent Byleth from turning into an eldritch horror.

Sakurai… What have you created?