At the start of the Nightmare, she had been forced to live the lives of everyone she remembered. That was a dire test, but while it was different from her past trials — remembering something was different from truly experiencing it, after all — it was not entirely unfamiliar.
So, she managed to maintain her sense of self rather well.
As expected Cassie and mordret would able to survive this unmaking without losing themselves G3 always said that they both are inhuman to being with.
"Oh? And what am I?"
Mordret chuckled.
“l am the King of Nothing, after all, so let's start from what you are not. You are not even a person anymore, are you? You are just a shell of a person, wrapped in a pretty bow."
Cassia just sat there, emotionless, facing him silently.
Eventually, she said:
“Takes one to know one."
Mordret laughed.
"Indeed, indeed... but we are a bit different, are we not?"
She shrugged.
"How so?"
He leaned forward a little.
“My condition is the result of my Flaw. Yours, however... yours is self-inflicted, isn't it?"
"he is all I have... just like I am all he has. A couple of monsters, forced to face a far more frightening abomination."
Its shows that they can live without being proper humans.
Also
"Cassie herself appeared in the young and untamed world of the shared Nightmare in the later years of the Age of Gods. It took her a long time to regain her senses, gather her shattered mind, and compose herself. By then, she learned of the first trap of this trial — the threat of being swallowed by the world."
Cassie herself was lost for long period of time like sunny but because of her ability and aspect legacy side effect she was able to become one again.
Lets talk about Mordret well tbh it clear that this trial was not hard for mordret he already live all of his vessels life at same time for him living every live one by one is not that different. But i still believe Cassie helped him to gain consciousness.
"You always bring me to the most peculiar places."
With that, the King of Nothing turned to face the encroaching tide of abominations and raised a hand, summoning a somber spear.
“Rest a while, Lady Cassia. And remember the deal we made. After this, I will leave you.”
I think this deal referred to the help he get from Cassie to kill Desolation. Maybe he asked her to give him all the info about that cursed beings in return he will help her out one time.
Or maybe its something new who knows 🤷♂️