The Dark Man by Robert E. Howard

When Viking raiders attack an Irish castle and kidnap a chieftain's daughter, the exiled warrior Turlogh Dubh O’Brien vows revenge. Braving a perilous winter sea alone in a small boat with only a mysterious dark idol for company, Turlogh pursues the raiders' trail to their stronghold on a remote island. There, with his life at stake, he launches a daring assault to rescue the girl from a forced marriage. But Turlogh soon confronts greater mysteries and darker powers than he expected... A thrilling historical sword-and-sorcery tale from the creator of Conan the Barbarian!