User blog comment:B214/Volume 23 discussion blog/@comment-172.56.16.153-20170417140249/@comment-9595671-20170417173707

I think that's in the sense of self-revival, Fafnir's page states he actually was killed once and the Norse Gods were able to revive him, and apparently without the Sephiroth Graal since no one mentioned it was used to bring him back.

It may also depend on the power of the particular Dragon, Apophis and Azi Dahaka have both reached Heavenly Dragon level strength, abd the Evil Dragons are all quite powerful. So it could be possible if Fafnir needed aid, he wans't strong enough, or atleast to enough to revive as soon as he was.

I guess the answer to that is still a bit vague, but Fafnir seems to show that a Dragon can atleast be revived much easier than any other species in the DxD world atleast.