A class of malevolent spirit from old quarian legend.
Dar'gheli, also known as D'Ghel and other variants, were considered to be shadowy spirits that dwelled near oases in the desert at twilight, waiting for a lone quarian to be isolated from their friends and vulnerable; then they'd strike and drag their victim down into the water.
In reality, the myth originated with bands of brutal desert bandits. Their preferred method of taking a travelling caravan was to set up by an oasis disguised as merchants and wait for travellers to stop by. When a group with enough supplies arrived, they would then select a member that closely resembled one of the acolytes and kill them by strangling or drowning them during the night, dumping the body in the oasis to avoid leaving any traces behind. The caravan would subsequently move on, ignorant of the enemy in their midst. The infiltrator would wait until he was assigned as watch, then signal for his compatriots (who were quietly following behind) to descend upon the camp, kill the caravan crew and take their supplies.
Of course, when the bodies of the first targets reappeared floating some weeks later, discovered by a new set of travellers at the oasis, showing no signs of violent handling but obviously part of an old caravan, speculation abounded.
Superstition soon held that you didn’t leave a camp to get water by yourself, for fear of being taken under by a Dar’gheli. It was like this until the Federal Period, when a process of governmental unity began between the cities, and effort was spent on hunting down the bandits in the hills.
The ‘Dar’gheli’ were driven into extinction, exposed as nothing more than common criminals - efficient, but far from supernatural. Nonetheless, the legends remained, with the added twist that once a targeted quarian was dead the spirit would emerge and appear just like them, a doppelganger, and would assume their life - bringing misfortune and sorrow to the people around it. When the quarians were driven off of Rannoch, the myth resurfaced in an altered fashion, with D'Ghel being quarians who had helped the geth on the homeworld and had been denied access to the next world as a consequence. As a result, they haunted the ships in the dead of night, appearing as a suit with nothing but a shadowy void inside it, just waiting for a chance to take another quarian's suit and drain them dry, taking over their old life and trying to reclaim an existence once again.