The Rhine Maiden is a massive anti-Gnosis sonic weapon developed by Vector Industries and constructed on its three main flagships, Woglinde II, Wellgunde and Floßhilde (Flosshilde) as well as its corporate headquarters, the massive space colony Dämmerung.
The weapon is capable of clearing large swarms of Gnosis by firing a pulsating distortion beam of some sort that breaks the Gnosis' physical forms down into their base element; salt. The Rhine Maiden appears to be harmless to humans but is fatal to all types of Gnosis. The weapon apparently can be fired from hyperspace and affect targets in real space, as a blast was observed clearing out a swarm of Gnosis that were attacking the Kukai Foundation long before the Dämmerung and her escort vessels had gated out of hyperspace. It apparently takes all four ships concentrating their fire in a triangular pattern in order to successfully produce the Rhine Maiden's effect. This may or may not be true, as the Rhine Maiden has only been seen used once so far in the series and it was done by using four vessels firing in unison.
In Xenosaga: The Animation, the Dämmerung was shown firing the Rhine Maiden without any firing effect from the Woglinde II, Wellgunde and Floßhilde, though this may have been an oversight on the part of the animators.
The names come from the Ring of the Nibelung in which the Rhinemaidens are three naiads who guard the magic Rhinegold lying at the bottom of the Rhine River.