Mr. Vane, the protagonist of Lilith, owns a library that seems to be haunted by the former librarian, who looks much like a raven from the brief glimpses he catches of the wraith. After finally encountering the supposed ghost, the mysterious Mr. Raven, Vane learns that Raven had known his father; indeed, Vane's father had visited the strange parallel universe from which Raven comes and goes and now resides therein. Vane follows Raven into the world through a mirror (this symbolistic realm is described as "the region of the seven dimensions", a term taken from Jacob Boehme).
Inside the world, Vane learns of a house of beds where the dreamers sleep until the end of the world in death: a good death, in which life is found. Vane's grandfather refused to sleep there and is, instead, forced to do battle with skeletons in a haunted wood. After a treacherous journey through a valley (where the moon is the only thing to keep him safe), Mr. Vane meets the Little Ones, children who never grow up, remaining pure children or becoming selfish and getting bigger and dumber, turning into "bags" or bad giants. After conversing with Lona, the eldest of the children, Mr. Vane decides to help them, and sets off to gather more information, although the Raven (who is also Adam) has warned Mr. Vane that he needs to sleep along with the dreamers before he can really help them.
While on his journey, he meets Lilith, Adam's first wife and the princess of Bulika. Vane, although nearly blinded by Lilith's beauty and charms, eventually leads the Little Ones in a battle against Bulika. Lona, Vane's love, turns out to be Lilith's daughter, and is killed by her own mother. Lilith, however, is captured and brought to Adam and Eve at the house of death, where they struggle to make her open her hand, fused shut, in which she holds the water the Little Ones need to grow. Only when she gives it up can Lilith join the sleepers in blissful dreams, free of sin. After a long struggle, Lilith bids Adam cut her hand from her body; it is done, Lilith sleeps, and Vane is sent to bury the hand; water flows from the hole and washes the land over. Vane is then allowed to join the Little Ones, already asleep, in their dreaming. He takes his bed, next to Lona's, and finds true life in death.

Question: What does Lilith give up to join the sleepers?
Answer: Water for the Little Ones.
Alternative answer: Lilith needs to give up the water the Little Ones need to grow.