I am being completely straight with you. If any of my recruits told me they were Ardat-Yakshi, I would immediately unholster my sidearm and put a round in their skull. No joke, no hesitation. If they actually are one, then I saved myself a lot of trouble. If they’re stupid enough to claim to be one, without actually having the disorder... I also saved myself a lot of trouble. -- Penumbra.
Literally translated, ‘Demon of the Night Winds’ ("Ardat" = legendary creatures compared to demons, "Yakshi" = "night winds"); the name applied to a rare genetic condition among the asari.
Although the condition doesn’t harm the sufferer, during mating (which in asari involves a melding of the couple's nervous systems) the Ardat-Yakshi’s nervous system completely overpowers and dominates that of her mate, causing haemorrhaging in the victim’s brain and ultimately death in extreme cases. The Ardat-Yakshi, though, becomes smarter, stronger and deadlier after each encounter.
The condition is impossible to identify until the asari reaches maturity, by which time it’s too late to correct. When diagnosed, afflicted asari are given a choice: live in isolation, or be executed. The asari do this to the Ardat-Yakshi because it’s an addictive condition; they feel compelled to mate, and the compulsion grows stronger each time they succeed.
While seductive and sexually-driven, Ardat-Yakshi are sterile. Contrary to popular belief, Ardat-Yakshi are neither extremely rare (around one per cent of asari dwell on the spectrum), nor are they all murderers. Most cultivate and discard countless exploitative or abusive relationships during their legally marginal lives. Despite rumours of Ardat-Yakshi syndicates, by nature Ardat-Yakshi are incapable of long-term cooperation.
In Wider Galactic Culture
As a disproportionately wealthy species, asari employ their economic reach and media ownership to hide the A-Y pathology from the galactic community, placing most Ardat-Yakshi in monitored work programs or seclusion. The most aggressive cases are sentenced to sanitaria and prisons, or to the execution lists of Justicars.
Ever since the greatly popular Galaxy of Fantasy added them as a playable character class, the Ardat-Yakshi mythos has become somewhat less obscure outside the Asari Republics, at least among gamers. That portrayal, as a sort of biotic super-witch, is no more factual than the turian mythology the game started out with -- it largely dates back to the Malari folklore, and the terror tactics of the so-called Malarial Queendom which used that canon for its own political ends. (During Thessia’s first industrial age, before the advent of spaceflight, a group of fanatical asari established a fascist state in the Kendra Ocean region, dubbing themselves the Malarial Queendom and claiming to be Ardat-Yakshi.)
Ardat-Yakshi antagonists in wider galactic media are often depicted as unaccounted-for survivors of the Malarial Queedom empire, or newly-born witches seeking to recreate or emulate that regime.
See also: Ardat-Yakshi Justicar.
Trivia
According to one asari legend, Eshil, goddess of Chaos, tricked another goddess into creating the first Ardat-Yakshi. T’Lanos, one of the legendary figures of early Justicar lore, is said to have retrieved the sword Cales from the bottom of Lake Bisel, symbolising the asari people taking upon themselves the fallen goddess’s duty to bring justice to the world.
A turian shopkeep at the Zakera Café sold asari honey marinades, which he said were created by sufferers of Ardat-Yakshi at a monastery. Similarly, according to the krogan known as "Patriarch", the asari honey mead sold at the Afterlife is Aria T'Loak’s most expensive wine and is brewed at an Ardat-Yakshi monastery by “crazy blue women”.
When Niamh Callaghan and Paroc Didonas were on Omega searching for Korman Trulion, an asari they overheard had the gall to suggest that Aria T’Loak was an Ardat-Yakshi.