The Scroll of Lucifer(1990)
(The fourth book in the Gates of Lucifer series)
A novel by Lloyd Arthur Eshbach
Three times Alan MacDougall had passed through the strange Gates in the ruined Scottish tower to find himself in the worlds of Lucifer, where the demon-gods lived on. There, unknown to him, Ahriman ruled as lieutenant to the Prince of Evil--and sought to entrap Alan into serving his Master's foul plots of conquest. Now the fourth Gate waited. Alan's only friend in Lucifer's twisted lands, the Bard Taliesin, had already passed it. But Alan lingered beside his beloved Eslpeth. Then he stepped through. This time, as he and Taliesin made their way through the realms of the fourth land, the dark magics of forbidden mysteries were shown Alan, but nothing he could use against Ahriman's determined effort to force Alan to lead Lucifer's legion to universal domination. When Alan sought to flee back through the Gate, he found that is had vanished. For him and Taliesin, there was now no escape!
Used availability for Lloyd Arthur Eshbach's The Scroll of Lucifer
April 1990 : USA Mass Market Paperback