Doom Cave is the cave which had an interior cavern which leads to a little island with a poisonous lake in the middle in which lies the "Toxic Tree" where the Fruit of Venom (The Evil Queen 's talisman) lies. After Evie got her mother's talisman, the lake began sinking and the island began breaking apart which left the four no alternative but to go through the image of a door (newly created through magic) that was in the face of the tree.
While inside Doom Cave the four had to cover their mouth and nose to prevent breathing in the toxic air. Next they had to jump from one stepping stone to the next in the middle of the poison lake until they reached a tiny island with one apple tree (the Toxic Tree).
Evie (without aid) had to climb the tree and find the golden apple which was the "Fruit of Venom." When Evie found the apple she was tempted to eat it. Her resistance to poisonous apples (something she had built up from accidentally eating some in her mouth) caused Evie to fall asleep rather than die.
While in her sleep she drempt that she was alone in her mother's, every jealous, insecure and mind poisoning thought went through her head. The mirror in her mother's room showed images of Mal and Maddy laughing at her and pointing. The Magic Mirror also showed Mal, Jay and Carlos leaving her for dead alone at the Poisonous Lake. Finally she was shown the "Castle Along the Way" where she and her mother had been exiled to causing her to be void of contact with anyone but her mother and the spiders. The Magic Mirror showed these images in an attempt to convince her that she was friendless and that no one liked her. It made her want to show others what it would be like to be alone, abandoned and friendless. Though the apple tried to corrupt her dream with these images, Evie was able to fight off these effects and remind herself she was not friendless and that these were images.
The dream caused by the apple found in the Doom Cave was a test of her mind, it tried to poison her mind so that she would turn on her friends. By overcoming the thoughts the apple tried to place, Evie became the master of the talisman so that when she woke up she had the apple (no longer glowing but regrown) with her.