The Third Prophecy was the prophecy given by A-dui to the Immortals that spoke of their fate and their legacy. The First Prophecy, the Second Prophecy are believed to have come to fruition already, and many believe that the Third Prophecy is being fulfilled in our own time, while the Fourth Prophecy will only come to pass at the end of the world.
Immortal you may be, but your time here is not forever. You shall pass beyond here and await My return, yet your blood shall reappear on this world when its need is greatest.
Before now there were different interpretations of what the second part of this Prophecy meant, but now, with the appearance of The Blood-Marked most people believe it refers to them.