The Souba is a katana with two reinforced parallel blades, set a small distance apart. The design's intent is to trap an opponent's blade between them and disarm them. Kadaj wields Souba as a normal sword when confronting Cloud in the Forgotten City, but fights holding it in a reverse grip when fighting Cloud in Midgar. Despite it being much smaller than the Fusion Swords, Kadaj battles Cloud with the Souba with no apparent handicap, taking advantage of the dual blades' wider striking range to more easily deflect Cloud's attacks.
The blade is lost in the ruins of Midgar during his final battle with Cloud, but after Kadaj reforms into Sephiroth and is subsequently defeated, it regenerates with him. Following his death in Aerith's Great Gospel, both Kadaj and the sword vanish into the Lifestream.
Like how Kadaj is a Remnant of Sephiroth, his weapon can be viewed as a segmented and salvaged version of Sephiroth's signature Masamune. Kadaj carries a specially designed scabbard for the sword across his lower back; two parallel sheathes joined by a block between the mouths of the objects.
Souba is Japanese for "twin blade".