To unlock it, you must complete a game in Soulcalibur Legends, restart your game and go up above your rank of Knight, reaching the third-best ranking. If you succeed, you will get to the weapon.
Pandemonium is the capital of Hell in Paradise Lost, an epic poem written in the seventeenth century by the English poet John Milton. The term is derived from the Greek ("παν", meaning "all" or "every", and "δαιμόνιον", meaning "small spirit" or "little angel". It was then used by the Christians with the meaning of "little devil", and then "demon"). Translated literally, the words means "all hell".