As Siegfried journeyed with his friends, he gathered splintered shards from Soul Edge and fused them to his sword, making it more powerful. As it grew in strength, it transformed into this shape, becoming a much more dangerous weapon.
It first appeared in Soulcalibur III, where it was Nightmare’s 2nd player costume default weapon, as "Soul Edge (Phantom)".
The Soul Edge Ω in Soulcalibur II, as the Player 2 variant of the Soul Edge (Phantom)
Nightmare wielding the Soul Edge Ω in Soulcalibur III