The Tale of the Outcasts, Volume 6 (The Tale of the Outcasts #48-57)

Two immortal great demons have each chosen a human companion to pass the endless years. Marbas' choice was a lonely girl, while Dantalion's was a bloodthirsty maniac. Their pacts with these humans began as mere diversions, but developed into unique relationships over time. When their chosen paths bring them into direct conflict, each demon's pride is on the line.