Celeste Cuckoo is one of the three remaining mutants known as the Stepford Cuckoos. She was also chosen by the Phoenix Force to host it's power and destroy the laboratory known as the World. When the Phoenix manifested itself in her, her and her two sisters Pheobe and Mindee started acting weirder than before and gained new abilities such as telekinesis.
This new power scared the Stepford Cuckoos and Celeste started to feel separated from her sisters, Which worried her greatly. After becoming the Phoenix she helped to destroy all the other Cuckoos and sealed the Phoenix Force inside her and her sisters diamond hearts.
Celeste Becomes the Phoenix.
The Stepford Cuckoos are identified as Weapon XIV (each girl being units 1 to 5, ordered alphabetically by their first names), the living weapon code between Weapon XIII (Fantomex) and Weapon XV (Ultimaton). Manipulated by both the Phoenix and Sublime, the Stepford Cuckoos awoke their dead sisters and traveled to the World, a laboratory created by Weapon Plus with technology capable of manipulating time, where Fantomex, Huntsman and Ultimaton were created.
During the sisters' visit to "The World," they were greeted by a computer image of John Sublime. It is revealed that the Phoenix fragment has manifested in order to destroy the Stepford Cuckoos and all of their clones to prevent them from being activated as a weapon and kill all mutants - Sophie and Esme being the first such casualties of its purge. Emma, Mindee, and Phoebe were successful in stunting Celeste's linkage with the Phoenix; only to be ambushed by Sublime's machinery, which absorbed the cosmic fragment and distributes it among each of the cloned telepaths. Now referring to themselves as the Thousand-in-One and under Sublime's control; the cosmically-empowered psychics proceed to enact their programmed destiny of mutant destruction. Celeste however, is still in partial control of both herself and her sisters' supermind, alerts the X-Men that they can short-circuit the girls' linkage by destroying Sublime's machinery; which they do. Phoebe, distraught at having lost access to the cosmic power, sends a bolt of her last remaining energy through the facility's floor... prompting an explosion that would destroy everything within a half-mile radius. Celeste, at Emma's behest, accepts her role as a Phoenix host, and stops the explosion by temporarily freezing localized time.
She then goes on to destroy the thousand-strong Cuckoo clones, by shattering their newly-manifested Diamond forms; before casting the entity out of her body. It however, refuses to depart the area; to which Celeste reacts by reabsorbing force into her own and her two remaining sisters' diamond hearts. Unlike their mother's diamond composition, the Cuckoo's hearts haven't a single flaw; and as such, nothing can destroy, enter, or escape them... even the splintered Phoenix itself.
The price the sisters must pay for this, is that they can never again feel emotion; thus leaving them even more cold-hearted and detached from their fellow man, than they were before. The girls are seen helping Elixir gain knowledge on Science via Beast's brain. They, too are teleported to Limbo with the other students and captured and have helmets placed on them to keep them asleep and to stop them using their powers.