The Emperor's Ring: The Pegasi Chronicles Book 4

Save himself or save humanity? Hell of a choice for a twelve-year old.
The immortals, who serve the Dark Lord, are relentless in their efforts to eliminate the human child who has the potential to prevent them from destroying humanity.
The immortals now have a powerful human ally – a man whose greed and thirst for power could spell the end of humanity in both this world and in the parallel world.
The boy can stop them – by using the power of a mythical ring that has been lost for over three hundred years. According to legend, the one wearing the ring can exercise great power, power to stop the spread of evil. The human boy is destined to be the one.
The price for that power – his life.