User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-5110811-20150606161023/@comment-26106956-20150610070926

there is also the fact that the darkness sort of seems like it is going to be an abstract or spiritual enemy, Leviathan were all physical the head leviathan couldn't even see a ghost so maybe God knew that they weren't suited to help in his fight against the darkness. also besides knowing that the darkness predated god, and eventually went to war with god and the archangels, we don't know what they're relationship was like before. So it is possible that god had made things before and the darkness ended them making an enemy of god, who would create and lead the archangels against the darkness.

P.S. All we know is that death said that leviathan were older that angels. archangel are still angels, as are the grigori, seraphs, rit zen, and even reapers now, so just because they have a fancy title doesn't mean that they aren't angels.