It is the common knowledge that, it was Prabhas (Dyaus), one of the Vasus, incarnated as Bhishma due to curse. Following passage from the Mahabharata supports it.
From this chapter of Sambhava Parva:
But he for whose deed ye have been cursed by me he, viz., Dyu, shall for his sinful act, have to dwell on earth for a length of time. I shall not make futile the words I have uttered in wrath.
But in one of the later chapters, the Vasus make another statement that, 1/8th of all eight Vasus become Bhishma:
We shall each contribute an eighth part of our respective energies With the sum thereof, thou shall have one son according to thy and his wishes. But this son shall not begat any children on earth. Therefore, that son of thine endued with great energy, shall be childless.
Also in another occasion , during birth of Arjuna , Mahabharata explicitely mentions that , all 8 Vasus were there
And the twin Aswins, the eight Vasus, the mighty Maruts, the Viswedevas, and the Sadhyas, also came there.
So which one is right?