The Codex entry for Ardat-Yakshi remained unchanged between Mass Effect 2 and 3, so it is not a ret-con.
Ardat-Yakshi ("demon of the night winds") are asari suffering from a genetic disorder preventing conventional melding of nervous systems during mating. Instead, Ardat-Yakshi electro-chemically ravage their partners' nervous systems, in extreme cases leaving victims as vegetative invalids or corpses. Asari psychologists regard this incapacity for mental fusion as preventing the development of empathy, leading to psychopathy. There is no known cure.
The disorder generally begins in infancy, reaching full pathology during Maiden adolescent sexual development. While seductive and sexually-driven as other asari, Ardat-Yakshi are congenitally sterile.
Ancient asari mythology held Ardat-Yakshi as gods of destruction, depicting them as villains of countless legends and as the anti-heroes of numerous asari epics.
Contrary to popular belief, Ardat-Yakshi are neither extremely rare (around one per cent of asari dwell on the AY spectrum), nor are they all murderers. Most cultivate and discard countless exploitative or abusive relationships during their legally marginal lives. Despite rumors of Ardat-Yakshi syndicates, by nature Ardat-Yakshi are incapable of long-term cooperation.
As a disproportionately wealthy species, asari employ their economic reach and media ownership to hide the AY pathology from the galactic community, placing most Ardat-Yakshi in monitored work programs or seclusion. Only the most aggressive cases are sentenced to sanitaria and prisons or to the execution lists of justicars.
Here are the known population numbers for asari worlds revealed to the player while the Reaper war on asari space is in progress. Not included are worlds already destroyed/captured, nor do they include secret or unrevealed population figures.
Thessia 5,500,000,000 (orbital stations: 33,000)
Asteria 188,000,000
Cyone 260,000,000
Hyetiana 119,000,000
Illium 84,950,000 (orbital stations: 80,500)
Lusia 2,200,000,000
Lymetis 12,550
Nevos 677,000,000
Niacal 7,300,000
Phoros 43,700
Sanves 975,000,000
Zesmeni 620
Now take 1% of those known census figures, isolate them all on defenseless remote "gilded cage" prison colonies, and you have more than enough future Banshees to be harvested by the Reapers for the war and beyond.
Even if they do not have a lethal or full manifestation of the genetic disorder it does not matter, since Reapers can create a Banshee from any asari on the AY spectrum:
Banshees are the corrupted asari often found leading a Reaper strike force. The Reapers create them specifically from asari with active or latent predispositions to becoming Ardat-Yakshi, a rare neurological condition that enhances the asari's biotic power while causing the immediate death of anyone she mates with.
Lumbering as though in constant pain, the emaciated banshees are surprisingly durable opponents. They are devastating biotics able to hurl lethal balls of energy and create shockwaves as they regenerate. What Alliance military finds most disturbing is the Banshee's ability to spawn her own warp field and seemingly teleport during combat. Although their wails have no apparent physiological effect, the psychological impact is undeniable.
When banshees die, their Ardat-Yakshi genetics twist against them, causing a biotic implosion to ensure they evade capture.