When Jeanne's unnamed Anwyn lover was seen crossing the Annan Waters to meet her, he was apparently shot by the arrow (although this is not explicitly shown). This caused his death and eventually Jeanne's.
The device was designed to hinder Jeanne from leaving the waters. It also literally tore through the Realm of the Dead (the ROTD) causing damage, and they were unable to directly access Jeanne's records afterward. Other unnamed departments were also affected by this arrow.
The arrowhead does not have a regular sharp tip. Init is hinted that the tip instead has etheric properties.