Let's not forget that in the second phase against Deus Darnus in 1.0, Darnus has crystalized wings. S/he had those crystalline wings as s/he did not absorb the full aether from Dalamud. However when battling Deus Darnus in 2nd Coil, s/he has true wings and takes on the form of Bahamut then. I do believe the real Darnus did die after the first phase in 1.0, and the aether from Dalamud filled his deceased body with that aether, having him with those crystalline wings. That is why when Deus Darnus in 1.0 gives his Shakespearean pose of giving up his body, and thus turns to aetheral particles.
