Brian Lumley 
Transition of Titus Crow 

With the most powerful, and most evil, of the elder Gods, Cthulhu, roused from slumber and preparing to destroy all of mankind with a single blow, Titus Crow and Henri-Laurent De Marigny are hopelessly outmatched. But they are not hopeless- They have been joined by heroes from other times, other worlds, other planes of reality. In a marvellous climax, heroes from other Brian Lumley series gather for the dramatic final battle.