"The Demon, whatever it is, is suspected to be hollow. It is not known what lies within, but some suspect lightning. With the storms that herald its coming, and the way it moves and appears, this is not entirely unlikely."
my take on how Master Chief might be seen by the Covenant, with his reputation as the Demon
inspired by this tweet by Rythaze
more ideas/details/context below the cut
one thing that got me thinking was that the few of the Covenant who ever saw her probably see Cortana as a ghost (which is not entirely untrue considering her origins), perhaps a vengeful spirit of something they killed, and considering the blue blood of many Covenant species, they probably see all those lines & patterns as gory drippings.
as for Chief himself, the way he can just show up out of nowhere, move at unstoppable speeds, and disappear without a trace, even as far as once having suddenly appeared within the heart of High Charity, right in front of the Prophets, probably lends a lot to his demonic reputation, (and it'd add even more if they knew that was with the Gravemind's help!), and I'd imagine it causes them to specifically fear him in the same way that one would fear lightning
