Moros is a Greek mythological entity. Moros was the spirit of doom. In some versions of Greek myth, it is said that Moros is a brother of the Fates.
If we're going to follow this myth, Marethyu cannot be Fate. Instead, we can assume that Marethyu is working with the Fates in order to ensure that everyone will meet their fates; and that is death. Everyone dies; even Elders, Archons, Ancients, and Earthlords.
Perhaps this is why Marethyu is called the destroyer, because what he does is to lead everyone to their doom.
The Oberour Ar Maro (the henchman of death) looks very much like the Grim Reaper from the popular stories. He looks like a skeleton wearing a hood or a hat to conceal his face and holding a scythe. He collects the souls of the dead.
It is also worth noting from the quote above that the title "Marethyu" was derived from Oberour Ar Maro. This implies that the Archons knew him first before the Elders came to know him.