Book Euron would have made a great final villain for the show.
Winds Of Winter Chapter "The Forsaken":
"When he laughed his face sloughed off and the priest saw that it was not Urri but Euron, the smiling eye hidden. He showed the world his blood eye now, dark and terrible. Clad head to heel in scale as dark as onyx, he sat upon a mound of blackened skulls as dwarfs capered round his feet and a forest burned behind him.
“The bleeding star bespoke the end,” he said to Aeron. “These are the last days, when the world shall be broken and remade. A new god shall be born from the graves and charnel pits.” Then Euron lifted a great horn to his lips and blew, and dragons and krakens and sphinxes came at his command and bowed before him. “Kneel, brother,” the Crow’s Eye commanded. “I am your king, I am your god. Worship me, and I will raise you up to be my priest.”
“Never. No godless man may sit the Seastone Chair!”
“Why would I want that hard black rock? Brother, look again and see where I am seated.”
Aeron Damphair looked. The mound of skulls was gone. Now it was metal underneath the Crow’s Eye: a great, tall, twisted seat of razor sharp iron, barbs and blades and broken swords, all dripping blood.
Impaled upon the longer spikes were the bodies of the gods. The Maiden was there and the Father and the Mother, the Warrior and Crone and Smith … even the Stranger. They hung side by side with all manner of queer foreign gods: the Great Shepherd and the Black Goat, three-headed Trios and the Pale Child Bakkalon, the Lord of Light and the butterfly god of Naath.
And there, swollen and green, half-devoured by crabs, the Drowned God festered with the rest, seawater still dripping from his hair. Then, Euron Crow’s Eye laughed again, and the priest woke screaming in the bowels of Silence, as piss ran down his leg. It was only a dream, a vision born of foul black wine."
The rest of the chapter can be read here:
TWOW Excerpts - The Forsaken (Aeron I)