My submission for Darksiders Your Last Days Contest, I hope you all like it.
In my fictional version of the apocalyptic moment, theres only one human survived. As the war between Heaven and Hell reaching its end, both Angels and Demons take numerous casualties. The toll of this war worried the Archangels and they decided to cease fire against Demons, which brought a new chapter on this prolonged conflict. Both sides felt the loss of war, and a tragedy for their own future. Theres no man an Angel can protect, and theres no man a Demon can tempt anymore.
So they save the last human offspring, and send their best to protect him. Angels sent one of their Seraphim, while Demons sent one of the Fallen Angel. Both merged into one new being The Dark Seraph. His duty: Protect the human child for all cost, for if any harm was done to him, humanity will not prevail.