Aife was the wife of the Erathi King Lir, who lived with him and his two sons in what is known now as The Blackened Hall. She charmed not only him but everyone she met, as indicated by Scribe Pearse's journals. But also evident to everyone was her ability to "create wonders", such as her ability to summon great flames.
During her time in The Blackened Hall, more signs of her secret identity were made evident, as with the death and disfigurement of the Ever Glen, a magical forest granted to King Lir by the Hironar. What had once been a beautiful, life-giving wood turned into a treacherous "beast" of thorns and weeds.
Some point after the death of her stepsons, which she didn't seem fazed by, she eventually revealed herself to have been a Niskaru demon in disguise. The motive behind her actions is unknown, but after killing King Lir, she went on to murder the entirety of Lir's court.
The legend goes that she was and is an unkillable demon, who upon several fellings by the guards of Lir's kingdom, rose again simply to continue her murder spree.
Aife caused the destruction that gave The Blackened Hall its name. By the end of the kingdom's existence, Pearse writes that the halls are full of thorns and flame, two such things that can be traced right back to Aife - her ability to wield flame as Niskaru do, and what she caused to the Ever Glen.
While Scribe Pearse's journals only clearly state that she was responsible for Lir's death, it is likely that the deaths of her "stepsons" was also at her hands, given her ambition to destroy the Erathi kingdom.
Aife and Lir's tale almost certainly comes from the Irish legend "Children of Lir", in which Aife was the second wife to Lir, and, jealous of his children, had them turned into swans, and in turn, she was turned into a demon. Interestingly enough, the tale also makes mention of a people known as Tuatha Dé Danann.