H. P. Lovecraft's The Old Ones.

In 1930, sea captain Russel Marsh saw the light, and for 93 years his body was not his own. Inhabited by a Great Old One, he committed unspeakable acts in the name of the Esoteric Order of Dagon. Now free, he is in search of a way to go back in time to reverse the horrors wrought upon the world. But the cult has other plans and will stop at nothing to destroy him.