A Banshee is a horrifying creature that was mutated from asari. They have a special ability which enables them to emit high-pitched screeches. The asari that the Banshees mutated from are only from a unique group known as Ardat-Yakshi, asari with a rare neological condition that can enhance their powers, however, they can cause immediate death to anyone that they mate with.
The Banshee are terrifying to look at and are very formidable to fight against. Only those strong-willed and unwavering courage, like Commander Shepard, would stand a chance against them.
The Fayelons Story[]
The Banshees make their appearance again in the story. However, unlike the Banshees in Mass Effect 3, these were formed from Dark magic, making them beings of pure Darkness. It is a relief that these are not mutated asari, however, these are shown to be a lot more powerful than the ones mutated from the Ardat-Yakshi. The girls are able to stand a chance against them, but they are shown to be an incredible match, almost too much for them to handle.