The Forest House is the 4th installment (in-universe chronological order) of the Avalon Series written by Marion Zimmer Bradley. Published in 1994 it is the telling of Eilan's story, priestess and Lady of the Forest House (Vernemeton).
In the early days of the conquest, when the Roman Legions are aggressively persecuting the Druids, the sanctuary of the Goddess on the isle Mona is destroyed and its Druids are murdered and its priestesses are raped. The raped priestesses that conceived children killed all of the girl children but leaved the boys alive that were born and then killed themselves rather than live with the atrocities done to them; the males later became a rebel group known as the Ravens, which swore vengeance against Rome. Lhiannon, one of the remaining priestesses, re-establishes a new sanctuary in Vernemeton (Most Holy Grove) or The Forest House which is partially controlled and protected by the Romans.
The novel tells the story of Eilan, daughter of a noble druid family. She hears the calling of the Goddess and is chosen to become a priestess at Vernemeton, and later to succeed the dying Lhiannon as High Priestess. However, before her calling, she hears the voice of her heart, and during the magic night of Beltaine, conceives a son with Roman officer Gaius Macellius, son of the high-ranking Camp Prefect at nearby Deva. Gaius is an inheritant of royal blood through his Celtic mother of a southern tribe, the Silures. Eilan knows their son, Gawen, who shares blood from the Dragon (Celtic royal), the Eagle (Roman Empire) and from the Wise (Druids) will play a crucial role in Britain's future and accepts high sacrifices to protect his youth.
A major shift in the balance of power is in the air; Eilan senses that the death of her peace-loving Arch-Druid grandfather will cause it. She tells her friend Caillean (who was rescued from her uncaring mother in Hibernia by Lhiannon) to take a group of young priestesses to the isle of Avalon to found a new sanctuary and become the first high-priestess of Avalon. In Vernemeton Eilan is increasingly pressured by the new Arch-Druid, her father, to stop promoting peace and collaboration with the Romans. In a dramatic showdown she sacrifices herself (along with her love Gaius) to avoid a bloody insurgency and, in particular, to save the life of her son Gawen.