"If you truly exist, aid me now!"

Mythal was an actual ruler during the days of the Elven Empire.

If memory serve, nobody ever saw the old Gods, except in dreams, or talking through the fade, and I guess once they got turned in Archdemon.
And there's that whole Golden City Field Trip, allegedly Dumat said "Go ! It's gonna be great" and well...Although there are talk of the old gods going silent before that.
Anyway a number of things could be talking through the fade. Maybe that whole Dumat thing was a prank.

What

Yeah that's why he wanted the warden's he had unde rhis conrtol by hi ssie so he could jump right then i fthe quizzy had managed t ocatch up t ohim and attempted to corner him. So there he a tleas thad a fall back plan he didn't have on ereally when you corner him at th etempl ean dals with his pet dragon dead he pretty much had nowhree lfet t orun. H edidn' tneed the dragon so much to jump bodies thoug hbecause remembe rhe hadalerad yshowed he coulfd jump when we firs tfind him as Hawke in DA2 we jus tdidn't know he'd jumped into Larius/Janeka' s bodies. We only figuered  that out later when we discovered he had this ability so he didn't need the dragon the dragon just made it easier to pull off.

There are a lot of strange contradictory things said about the Old Gods, particularly in the Nightmare demon's realm. For example, the memory of the priest when the Arch-demon surfaces. He seems to know it is Dumat but how if they have never seen him? Does he just assume that a big powerful dragon spewing spiritually corrupt magic has to be an Old God? Remember this was meant to be a memory at the very beginning of the First Blight, so some 100 years before the foundation of the Grey Wardens, who might have been able to identify arch-demons by their connection to them.

Then there is the memory of Corypheus' servant. Now according to everything we were told previously, worship of the Old Gods was meant to be at its height just before the First Blight but according to the servant Corypheus embarked on his mission to the Golden city because they were losing worshipers, so it would seem Dumat had suggested coming to see him in person would give Corypheus the sort of status that would win back the faithful. Assuming of course that the voices in his dreams were the Old Gods. I have wondered if in fact the gods had stopped communicating before the ritual, this was why people were exhibiting a lack of faith and so Corypheus went to the Golden City to try and re-establish contact. However, when Hawke woke him he definitely complains "it was meant to be golden" in such a way that you feel someone (Dumat) told him it would be. Mind you, I assume that the city appeared golden in the Fade until they visited it and discovered the truth, so may be that is what he meant.

A question that I would like answered is where was Corypheus during the First Blight? The Grey Wardens only discovered him after it ended. Could it be that he was rendered unconscious by the "darkness" and he only revived after Dumat had been killed?

Anyway, if we ignore the anomalies in the Fade, Old God worship was based on contact via dreams but never in person. Presumably the reason they were worshipped as dragons was that if asked what they looked like, the priest was given the vision of a dragon. Also the first people to worship them were the Neromenians and they held dragons in high regard, even thinking their heroes were reborn as dragons, so may be they also believed that their gods would take the form of a sacred dragon. The Ancient Tevinter believed the Golden City was the home of the gods, never underground. So Corypheus definitely thought that he was going to meet his god, Dumat, when he went to the Golden City. Instead he found a Black City and an empty throne. So that alone should have given him reason to doubt the existence of Dumat. Which is also presumably why he thought a repeat visit to assume the throne of godhood wasn't going to offend any other deities because so far as he was concerned they no longer existed.

However, when facing final destruction, clearly he became desperate and made his plea, though why he thought Dumat would help him when he had been so faithless is anyone's guess. It is odd that he doubted he existed because if he knew the Chantry dogma concerning the Maker and also had had contact with the Grey Wardens, you would at least think he would know that they claimed to have killed Dumat. Therefore he had definitely existed by could no longer help anyone, so what would be the point of calling on him? Also, given that Erimond had been encouraging the Grey Wardens to think they should summon a demon army to kill the last two Old Gods, surely that meant Corypheus should be aware that these two Old Gods did still exist and if he was going to appeal to anyone it should have been them. Anyway, that final despairing speech of Corypheus was just that; the cry of someone who had totally lost faith in everyone including himself.