The Tulpa

An eldery man begins to develop psychic powers (he has visions of approaching disasters) even as his mind slowly rots away from alzheimers. His son-in-law investigates that phenomona and encourages the old man to try and create a stronger grip on those powers since they save lives. The old man's concentration leads to the creation of a Tulpa (a psychically controlled semi-sentient force that is projected from the body, or something like that). The Tulpa and its rampage does not appear until the closing pages of the novel, Williamson choosing to focus the story on the ESP investigating of the son-in-law and his daring rescues of those in his father-in-law's visions.