Blood of an Immortal by Michael S. Gardner
What is the cost of one man’s soul? This is the question which burdens a French assassin and a Spanish helmsman. An explorer has offered them the gift of immortality in exchange for their aid in wresting power from the tribe of cannibals that granted him his longevity. The souls of fifty men will be used as bait, and the reward will be an eternity of navigating the seven seas. But is the remuneration worth the risk?