Torment — Douglas Hill

Having succeeded in defeating the deadly Mr Redman, Nick and April are left alone with the knowledge that their only friends, Paddy and Julia, may be dead as a result. But as time passes April, whose psychic powers are increasing, begins to sense that they are still alive. So begins a journey in search of their friends. A journey for which even stronger magic is required. And a journey which takes them into a terrifying nether world where soulless beings walk a never ending path of mist and sorrow.
And if they find their friends . . . what then? Even with the help of creepy skeletal Bertrand do they have the slightest hope of escaping the path and returning home? Or are they too doomed to be swallowed by the mists and lost to the mortal world forever . . ?