Not my idea but it did make me go searching....
"I would like to combine folklore from all over the world. Maybe the Russian firebird legends." - Sarah J Mass
“They sold her—to … to some darkness, to some … sorcerer-lord …” She shook her head. “I can never see him. What he is. There is an onyx box that he possesses, more vital than anything … save for them. The girls. He keeps other girls—others so like her—but she … By day, she is one form, by night, human again.” “A bird of burning feathers,” I said. “Firebird by day,” Rhys mused, “woman by night … So she’s held captive by this sorcerer-lord?”
"Kastchei (also spelled Koschei) is the immortal sorcerer and primary antagonist. He imprisons innocent princesses and turns men into stone using his dark magic. The benevolent, magical Firebird helps the hero, Prince Ivan, defeat Kastchei by putting his evil subjects to sleep and revealing how to destroy the sorcerer's soul"
The Firebird: A magical creature of light and guardian spirit who is captured by Prince Ivan but offers him a glowing feather in exchange for her freedom (I wonder if this is any way connected to the deal Papa Archeron made for her temporary freedom).
Prince Ivan: The brave hero who enters Kastchei's enchanted garden and seeks to free the ten enchanted princesses.
Kastchei the Deathless: A sinister ruler whose soul is locked inside a magical egg hidden within a casket in his palace. (Palace could equal the castle that had enough spells and wards to trip up even Helion). Anyone who destroys the egg destroys Kastchei himself.
The Climax: When Kastchei's monsters attack Prince Ivan, Ivan waves the Firebird's golden feather. The Firebird casts a spell, forcing Kastchei and his horde to dance frantically until they collapse in exhaustion. The Firebird then helps Ivan destroy the egg, vanquishing the sorcerer.
Her voice hoarsened. “Koschei is no mere sorcerer. He’s confined to the lake only due to an ancient spell. Because he was outsmarted once. Everything he does is to free himself.”
“Why was he imprisoned?” Cassian asked. “The story is too long to tell,” she hedged.
“I fear what may happen if he ever gets free of the lake. If he sees this world on the cusp of disaster and knows he could strike, and strike hard, and make himself its master. As he once tried to do, long ago.” “Those are legends that predate our courts,” Eris said. Vassa nodded. “It is all I have gleaned from my time enslaved to him.”
Lucien stared out the window—as if he could see the lake across a sea and a continent. As if he were setting his target.
“Maybe not.” Eris shifted on his feet, and grimaced again. “But you and yours have more important things to think about than ancient history. My father is furious that his ally is dead, but he’s not deterred. Koschei remains in play, and Beron might very well be stupid enough to establish an alliance with him, too.
"Tell my Vassa I'm waiting"