DISCLAIMER: I do not own Soulcalibur or Bleach.
The human soldier hit the ground, groaning. The Azure Nightmare stood over him, raising the horrific blade Soul Edge. As Nightmare was about to crush Maxi, a man in a black martial arts outfit appeared between them, blocking Soul Edge with his katana. The man turned his head to look at Maxi.
"You should leave. This is a fight between spirits. Humans don't belong in this fight."
Maxi picked up his nunchaku, but Byakuya gripped Maxi with his free hand, tossing him away.
With Maxi gone, Byakuya turned to face Nightmare. Nightmare glowered at Byakuya, his eyes glowing brighter red.
"You interrupted my meal! I'll just have to eat you instead. Are you tasty?"
Byakuya crinkled his nose slightly in disgust. Nightmare and Byakuya shoved each other back, and Byakuya flash stepped forward, slashing Nightmare. Nightmare attempted to crush Byakuya, but Byakuya vanished, appearing at the other end of the destroyed field. He held up his sword.
"Scatter. Senbonzakura."
Byakuya's sword shifted into thousands of shards which resembled rose petals, and rapidly swirled around Nightmare. Laughing, Nightmare released a burst of dark energy, blowing the petals in all directions. Byakuya held up his sword as it reformed, before shaking his head. Nightmare charged, and started swinging Soul Edge at Byakuya. Byakuya was able to defend himself, but Nightmare was deceptively fast.
"You thought your party tricks would stop me, Soul Reaper? I have devoured hundreds of Soul Reapers in my time on Earth, but I believe your soul will be the tastiest! Tremble before the dark lord, you pitiful Soul Reaper!"
Byakuya flash stepped away from Nightmare, waiting for Nightmare to attack.
"You've obviously never faced a Soul Reaper captain, or you would not disregard us so quickly."
Nightmare started to slowly approach, dragging the tip of the imposing Soul Edge along the ground, leaving a deep furrow in the dirt.
"Oh, I've faced enough Soul Reapers, trust me. You are an insect to be devoured like a quick snack, and your entire race will eventually follow! I will destroy the Soul Society and this world too! My fires will consume all of existence!"
Byakuya held up his sword loosely.
"Seeing as you call us insects, and threaten to destroy us all, it is only fitting you see what a captain can do. Bankai."
Byakuya dropped his sword point first to the ground, and it vanished as if he had dropped it in water. Dozens of streetlamp sized blades appeared around Byakuya, and began to scatter.
"Senbonzakura Kageyoshi."
Nightmare laughed, releasing a burst of his soul based power to deflect Byakuya's Bankai.
After the attacks of the two warriors had been used, there was stillness for a few seconds. Finally, Nightmare's Soul Edge fell from his demonic hand, shattering on the ground. The armour that was Nightmare's body then exploded into thousands of pieces of metal, collapsing into a large pile of shards. The darkness that was Nightmare's core then burst, leaving only the shredded armour behind.
Byakuya sheathed his Zanpakuto, standing over his vanquished enemy.
"You had no right to look one such as me in the eye. Or to look any true soul in the eye, for that matter. You are an inanimate object that forgot its place. And now you are back where you belong."
Analysis:
While Nightmare is highly destructive, Byakuya is a far more skilled warrior, and there is no denying that he is far faster, and believe it or not, stronger. His strength was similar to that of Ichigo, who was able to physically hold his own against Kenpachi, whose feat of physically engaging Yammy Riyalgo's gigantic released state clearly places his strength well above Nightmare's, whose biggest strength feat is sending a large group of soldiers flying with a single swing. If that wasn't proof enough, Kenpachi's ability to jump miles into the air also places his strength well above Nightmare's.
While Nightmare is a soul blade and could certainly harm a Soul Reaper, as his abilities are demonic, which Spiritual Pressure is shown to be vulnerable to, Byakuya is one of the greatest warriors in the Soul Society, and would be superior in skill to the wild, almost irrational fighting style that Nightmare uses.
In closing, though Nightmare's abilities are soul based like a Soul Reaper's are, and his dark magic could indeed defend against spiritual attacks, Byakuya's Bankai has been shown to be able to swipe single targets to pieces, and has been shown to penetrate many things, including an Espada's skin. Nightmare's dark magic is used as an attack, not a shield, and would not be adequate to force away such a deadly attack as Byakuya's Bankai.
