What Sustains Immortals? Exploring Mythical Life Force

Your premise is not technically correct as the term "immortal" typically refers to mythical concepts. However, I can clarify that the concept of blood or its equivalent might vary depending on the mythical creature or the fictional world they belong to. In some portrayals, they may have a substance referred to as "immortal essence" or "vital life energy" flowing through their veins instead of blood.