The time eater

"When Roger Borough receives an unexpected call from a mysterious woman, he is summoned into the past by news that an old college friend is dying. When he encounters James Steiner again the floodgates of his unconscious become unlocked, releasing a deluge of memories and fears he has worked hard to forget. But that's not all. Their reunion also unearths a secret, a ritual from their shared past that awakened an entity so vast, so outside of space and time, that it shattered their youthful reality. Now that entity is back. And it is ravenous and all-devouring..."--