It is no secret that the DC Universe sometimes absorbs other characters from different publishers into its ranks. This is how the WildStorm Universe was merged with DC, adding a new dimension to the existing universe. Another merger occurred during the 2018 Justice League of America run. During a battle with the Queen of Fables, Tsaritsa, the League met and allied with Promethea, the titular character from the 1999 Promethea comic created by Alan Moore, J.H. Williams III, and Mick Gray. Her arrival was brief, but it established her as an extremely powerful character who was largely responsible for enabling the Justice League's victory over Tsaritsa.
Promethea made her reappearance at the tail-end of Justice League of America #23 by Steve Orlando, Neil Edwards, and Clayton Cowles. After Tsaritsa had been brought to back to the material plane with the aid of Killer Frost, she captured the rest of the Justice League and seemingly killed Vixen. Her end goal was to build a gateway to Immateria, the realm of imagination. Once inside, she planned to seize control over the minds of every living being in existence and remold them into a form she found more suitable.
The League did their best to stop her, escaping from their shackles to fight Tsaritsa and her army of Woodsmen. But they were unable to prevent the gate from opening. This was not the end however, for while Tsraitsa was desperate to enter the realm of imagination, she had ironically lacked enough imagination to conceive that there might just have been someone waiting on the other side of the threshold. And as Tsaritsa tried to enter this brand new world, Promethea stepped out of the gate and attacked her.
Promethea is the champion of imagination and creativity, first introduced as a girl whose family was attacked by a Christian mob in the 5th century A.D. because of their belief in the old gods. Promethea fled into the woods at her father's behest, hoping that their deities would protect his daughter as the mob killed him. His prayers were answered when Promethea was saved by Thoth-Hermes, who whisked the girl away to safety in Immateria, the realm of imagination. Here she would live eternally as not just a girl, but a story that would never truly die. Thus, Promethea was forever linked to Immateria, becoming the guardian of human creativity.
Centuries later, a young college student named Sophie Bangs was researching the myth of Promethea, whose name constantly reappeared in literature and pop culture. Her search lead her to Barbara Shelley, the current Promethea, who then passed on her abilities to transform into the legendary warrior to Sophie. As Barbara explained it, Promethea is called into the world when someone uses their imagination to make her real. Sophie is the latest in a long line of women throughout history who have taken up the legendary mantle.
And it seems that Sophie has maintained Promethea's status as guardian of imagination. She was unwilling to let Tsaritsa enter Immateria, deeming the Queen of Fables unworthy of such a place not only because she was evil, but because she lacked imagination. Her intervention bought the Justice League enough time for Killer Frost to see the error of her ways and stop Tsaritsa herself. It was sort of a deus ex machina, but it certainly made for an epic fight. And it also added another facet to the rich tapestry of DC's cosmology.