Narnia book 4. Eustace Scrubb, with a friend named Jill Pole, is sent by Aslan to find the imprisoned Rilian - the true heir to the Narnian throne. Guided by Puddleglum, the children help Rilian to escape from Underland.
The Silver Chair - Jill, Eustace, and Puddleglum are taken to the subterranean world of Underland by the Earthmen, who are servants of the Lady of the Green Kirtle, or the Green Witch, who was holding Prince Rilian (Caspian's son) hostage. Here they enter a cavernous chamber with hundreds of sleeping dragons and other similar creatures.
Narnia book 7. The final story: in the last days, a clever ape has constructed a false Aslan. Even after Jill Pole and Eustace Scrubb help Tirian to expose the deception, confusion reigns. The children die in a railway accident in England at the same time that Narnia ends. The children go on to find a new Narnia where "the inside is larger than the outside."