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.