Its in-game description reads "Phantom armband. Forged by spirits." As a armband it is quite effective against enemies like the Night Stalker and its variants, because they're weak to Monica's armbands. If you can, charge it, so you can finish them in only few shots. You can only find this armband in dungons' chests, it cannot be invented or bought in any way. This brassard is a medium-tier weapon (Tier 6) : it'll gain 4 Synthesis Points (SP) per level-up.