Lady of the Lake
The Lady of the Lake (AKA. That lying bitch!) is the patron Goddess of the Bretonnia army in Warhammer Fantasy. She demands nobility, honour, courage, and absolute steadfast approach to all things. In order to even BE nobility in Bretonnia, you must dedicate yourself to her (simple enough) and join the Knight Errantry after your squireship ends. Those who actually want to be somebody someday will eventually (temporarily) forsake their titles and property, becoming Questing Knights looking to wander the world and prove themselves to the lady. Those who impress her earn a boon; she appears before them holding a magical chalice, and after they drink from it, they find themselves buffed to demigod levels of strength. The knights who accomplish this are known forever more as Grail Knights, supreme badasses who it would not be unfair to call the Space Marines of Warhammer Fantasy. Of course, most Questing Knights die in the process (also like Space Marines).
For years The Lady was thought by the playerbase to simply be Ariel, the queen of the nearby Wood Elves manipulating the Bretonnians into doing their bidding. However, 8th edition confirmed that she is indeed a separate entity; possibly Elvish, the Wood Elves call her "Corrigyn, The Daughter of Mist". It's theorized by some that she either is or is the daughter of the Elf goddess Ladrielle who is also connected to mist. The End Times revealed she was in fact Lileath (who Ladrielle was confirmed to simply be an aspect of), and that her Damsels and Grail Knights would form the human pantheon in the new world.
She is described as having a home on the Silverspire; in the End Times, she takes the formerly-current king of Bretonnia, King Louen Leoncoeur, there to be her husband.
While she only appears before Questing Knights who prove themselves worthy, the Lady still maintains an active role in the lives of her worshipers. She chooses a mortal servant, always female, from the Bretonnian nobility called the "Fay Enchantress". The Enchantress sends messages to the nobility, and keeps an honored position in the king's court. The Fay Enchantress has supreme authority in the kingdom as her word is the word of the goddess and has in the past stripped nobility of their rank and possessions and had them exiled without question (with the Grail Knights doing the heavy lifting).
Any child born with magical talent in Bretonnia is visited by the Fay Enchantress upon reaching puberty. Before this time, some children are sent to the Empire to be taught magic in the Colleges of Magic. The rest are taken by the Enchantress to the "Otherworld" of which nothing is known. Culturally they are treated as dead, and the males never return from what the Bretonnians assume is a happy ending. Female children sometimes return however, and are known as Damsels or Prophetesses. They speak little of what they've seen and done, although they claim to have been trained by "Handmaidens of the Lady" in their magics. While they cast spells like ordinary Wizards might, their magic is primal; safer to cast due to coming from something not of the the powers of the Warp and reverse-corrupting what the Warp touches. Their magic is directly from the Lore of Life, although like the magic of the Queen of the Elves is not the type of Life that Wizards can use and wield via study. They are unapproachable in society, serving as advisers to nobles or tenders of the Lady's sacred sights as she sees fit to send them.
The current Enchantress is named Morgiana Le Fay, and in the End Times she was converted to vampirism then sacrificed to resurrect Nagash (possibly, as it is only mentioned in one of the two sources of the event).
If you didn't realize it by now, she's directly and unashamedly inspired from Arthurian myth. Except this time she's all over the French. (despite in Arthurian legends, the Lady of the Lake was in Northern France anyway)