Progeny by Shawn Hopkins
It has been months since John Carter's estranged brother, Henry, has gone missing. When last heard from, he was sailing off to Bermuda in search of an author whose books deal with the esoteric traditions of past ages.
Reluctantly, John joins Henry's old Special Forces Teammates on a trip to Bermuda, hoping to discover the truth behind Henry's disappearance. But not all is as it seems in Bermuda, and the puzzle that awaits John on the small island paradise will prove to be more sinister than anything the world has seen in a very long time.
As the fingers of an ancient evil seek to draw him into another world - a world where all of Hell is trying to break loose - John must confront the truth of his own past...and pray that he might survive its revelation...
...that the world might survive its revelation.