Weird Sisters (Gargoyles)
Trio of Oberon's Children - the mystic Luna (the Sister of Fate), the sunny Phoebe (the Sister of Grace), and the harsh Selene (the Sister of Vengeance) - who were assigned to guard Avalon after Oberon and the others left the isle in 995. The Magus was forced to battle them to gain entrance to Avalon, and defeated them by deflecting one of their spells and turning them into barn owls.
Incensed, they desired revenge on the wizard, and so were quite willing to ally with the Archmage in his plans. They manipulated the lives of Demona and Macbeth to create the perfect, immortal warriors, and by 1995, they took control of their minds and had the pair gather the Phoenix Gate, Eye of Odin, and the Grimorum Arcanorum - items that the Archmage needed to empower his past self. They also aided in the Archmage's assault on the Avalon Clan, and finally had their confrontation with the Magus- who defeated them by ensnaring them, but died in the effort.
The Sisters were freed by Goliath on the terms that they free Demona and Macbeth from their control. They agreed, did so, and left, but later returned with Oberon and Titania when they reclaimed Avalon. They tried to have Oberon destroy the Avalon clan and company, but Titania manipulated him away from that end. When the Gathering was begun, the Weird Sisters were assigned to retrieve those Children of Oberon who refused their master's summons, such as the Banshee. They also offered to hunt down Puck, but Oberon wished to do that himself.
Presumably, they are still functioning as Oberon's aides on Avalon.
Greg Weisman's Master Plan
According to Greg Weisman, the Weird Sisters were far from finished in their plans to gain revenge. Goliath and the Manhattan Clan would be in their sights as well.
Greg has also mentioned that the Sisters are not yet done with Demona and Macbeth, and as everything they do is three-fold, we'll see the grace aspect of it.