Its in-game description reads "Evil sword that presides over everlasting darkness." Like the Sargatanas, the Dark Excalibur has the slow 5-hit combo, but its got very high stats if you found this weapon. And with the special abilities obtained from the previous weapons (if you obtain this weapon with the build up system), it's even more powerful. You can only find the Dark Excalibur in dungeons' chests and therefore cannot be invented or bought. The Dark Excalibur is a high-tier weapon (Tier 7) : it'll gain 6 Synthesis Points (SP) per level-up.