The Darkling.

When mechanic Jeff Obold meets the wealthy and mysterious Bruno Rubin, he is exposed for the first time to the finer things in life. But Jeff soon discovers the source of Rubin's power: a caged creature called a "Darkling," which possesses the power to create incredible wealth for the person who will pledge unending "companionship." Now, as Jeff sinks further and further into the blackness of a soul sold to the devil, he will discover there is a high price to pay for the fulfillment of his dreams.