The Serpent and the Grail by A.A. Attanasio is currently free on Amazon (US, ES). This is the product description:
The narrator is Rna, a clan woman 90,000-years-old. She is eldest of the Nine Queens of Avalon.
The place is Roman Britain, A.D. 491 on an alternate Earth, where Camelot is a monumental quantum gate to other worlds—including the one world among all possible worlds where you join Merlin and Arthur inside Rna’s story…
Weaving together the hallowed lore of Arthurian legend and the marvels of quantum science, The Serpent and the Grail places you on the quest for the Grail.
Rna the Ice Age queen—beckons: “Why did we create this spell, our retelling of King Arthur's long-ago story and our news from Avalon, except to summon you? … We need your help.”
The boy-king Arthor has beaten back the berserker wolf warriors of the Saxon god, uniting his island kingdom—but at a terrible cost. The Grail, the chalice of the Last Supper and the magical shield of Britain, has been stolen.
Without the Grail's protection, Britain lies parched and barren, a demon-haunted landscape.
To find the Grail, Arthor's mother, Ygrane, must strike a dark bargain with Bright Night, elf-prince of the "pale people," in the hollow hills. Merlin alone can show the way. But it is Arthor who must confront the Serpent, sword in hand, to fulfill the ancient blood rite of kings—and transform himself to King Arthur.