The Lurker was a Fallen Lord that served under Balor. She was also called the daughter of flood and famine. Besides hypothetically having powers of a Mahir, Bheil could also use powers of water and starvation. It could be inferred that The Lurker had some special relationship to Soulblighter and Shiver. She is also called Twice Born a name given to Soulblighter. Intriguingly Soulblighter resurrected Shiver over any other Fallen Lord, and strangely Bheil bares similarity to both Shiver and Soulblighter. Despite this however, she is referred to as an "ancient evil," and should not be counted as being the child of Shiver and Soulblighter. The Lurker was a Fallen Lord and was one of the Sorcerers who attacked the West. She was most likely destroyed during the Great War--but given the amount of Fallen Lords that actually lived through the Great War, one can only assume.
"...convinced her that everyone was out to get her – everyone she loved, everyone she cared about. There were enemies lurking in the shadows, he told her, who would hurt her and make her suffer. She surrendered and pledged allegiance to Balor."
She is called the daughter of Flood and Famine, for she can bring those disasters upon the world.
"She, like her imagined enemies, took to lurking in the shadows breathing fear down the necks of her enemies."
"...has become but a fairy tale story, told to children that she lurks in the shadows, hides behind the doors and waits to spring for those who disobey. She resides in the nightmare world of sleep, coming out to strike fear. The truth is much more terrifying. She is the Bone-Splitter"
It is possible that Bheil was a Callieach that had done horrendous deeds and died. This would make sense as to the power she held and the reference to her being an "ancient evil." She probably aided Soroganth the Flayer in his attempt to crush the light.