Concept Art 51: Archon Adonaios (Patreon)

Adonai's archon form is a bit different from the other Archons. While Sabaoth and the others are supposed to look bestial and demonic, Adonaios looks more humanoid and there is a reason for that. Adonaios is the only archon/person who has achieved true synergy, a complete merge of consciousness, while the other midnight blades are still in lower levels of harmony with their archons.

The reason for this perfect merge was the unique situation Adonais was in. After the Absolute Zero event that destroyed the last vestiges of humanity, Adonai and the Archon was vulnerable to the closure of the Omega gate (the black hole), so in order to both survive, the original Archon completely merged with Adonai to create a resistant vessel which could withstand the effects of the implosion ( while the bodies of living beings became ashes due the heat of the implosion, part of their minds became errant entities that looked like shadows or ghosts whit, not physical bodies that eventually will be called the Nox, while the other part of their existence being their soul, were sucked by the black hole that stripped everything from its light.).

The ability of the Archon to infinitely regenerate Adoanai's body helped the man survive, and eventually, the Archon's consciousness merged with Adonais while they were on the brink of death to protect its mind and soul from the black hole, all that thanks to the dark polarity Magh. So, while Adonai conserves most of his mind intact, there is no denying that there is part of the original Iblis in him, that due to lack of ego just became part of Adonai.

His design reflects that perfect merge of both existences, making him look more like a fallen angel than a demon. This concept also represents Adonai's character who has nothing to hide and does not fear death anymore, that's why he exposes his body (both in his armor design and this design).  His only reason was to persist in the hope of making things right,  giving freedom of choice to the humanity he helped create and condemned under the control of Adam.

I guess you could say that while he is falling into madness, he is also trying to hold on to what's left of his humanity, which is reflected in this design. As a side note: nope, this is not how Adonai looked as Iblis back in the prehistory, this new form is the culmination of the merge with the Archon after he fulfilled his role as Iblis. You could say that any Archon, as a fragment of Adam, is a potential candidate to become iblis as long as the vessel is correct.