Sayge is the embodiment of truth. It guided Doctor Strange from the Dungeon of Doom, a place in the Dark Dimension where Umar would take her enemies and leave them to rot. It also helped Strange in his battle against Umar, distracting her during their fight.
He was an advisor to Anath-Na Mut the original Sphinx. When the original Sphinx would look into Sayge's face he would see his ultimate fate, presumably his being caught in a time loop by Galactus. He once offered to show Galactus his ultimate fate but Galactus, already knowing his fate, saw no need to see it.
Later when the second Sphinx Meryet Karim used the power of the Ka Stone to rewrite history, Sayge appeared to Nova and showed him the truth regarding the false world Meryet had created. Together Sayge and the New Warriors showed her the error of her ways and she restored the timeline to the correct version.
Sayge next helped the two Sphinxes realize their feelings toward each other and bore witness to them merging into a composite being and go back in time to relive their lives.
Powers and Abilities
Sayge's face shows the truth. He is not affected by revisions to the timeline. He seems ethereal and attempts to attack him do not hinder him.
The revelation that the character Sayge seen in Nova Vol 1 was the same being as the character Veritas seen in Strange Tales Vol 1 comes from the Official Handbook of the Marvel Universe Vol 1 #10. This was later confirmed in the comics in New Warriors #47.