I am like 90% sure that it was confirmed at some point that Jace went to Iquatana at some point (although maybe I’m just assuming it based on the presence of the phantasm?)
Okay, so let me see what I can do with Ashiok. (Forgive my shittiness at throwing together these cards in about two minutes.)
They hail from a plane where there are pockets of places where dreams become real, but those sites are sacred and no one is allowed there. Ashiok was intensely curious about them, but respected their culture’s wishes and tried to follow the shamanic path that their parents expected of them. Shamans on this plane are protectors of the dream-places but expected never to enter them. In fact, no one has been allowed to enter these places for hundreds of years.
At first, Ashiok was content with their life, but they began to question things. Why was no one allowed into the dream-spaces? Eventually, their curiosity got the better of them, and one night they snuck into the place they were supposed to guard. It was beautiful. They saw all the dreams of the people close by, taken form and were fascinated by them. Even by the nightmares, which were frightening but awe-inspiring. They came back.
One night they tried to modify the dreams and realized they could. They stopped one of their little sister’s nightmares. From then on, they started protecting everyone by stopping all the nightmares that they could, and their people flourished, because they always had wonderful dreams.
When a neighboring group of people tried to attack their village, Ashiok defended it with their nightmares, but the village found out, and instead of thanking Ashiok for protecting it, they were horrified that Ashiok had trespassed in the sanctity of their dreams.
They turned against Ashiok and tried to kill them, believing Ashiok had been controlling them and had defied the gods. Ashiok was beaten and dragged back into the village, where they struggled and pleaded, saying that they had only been trying to help.
The village elders disagreed, saying Ashiok had been doing it for their own glory and that they were to be executed. Just before the execution, Ashiok angrily denounced the village and called upon their dreams and nightmares to defend them. All the dreams and nightmares came to defend Ashiok, but at a cost they had not expected. The people whose dreams had been taken from them fell at Ashiok’s feet unconscious, and the dreams began to devour them, their limbs fading to insubstantial smoke as Ashiok watched.
Ashiok panicked and tried to make the dreams stop, but they turned on Ashiok as well. Ashiok could feel their mind and memories flowing away, but as they began to lose themselves, something new inside them awakened.
They were flung across the Blind Eternities, landing on a quiet, dream-like plane where they could learn to take control of their newfound abilities.
(Some of the card templating is just plane wrong–I threw these together and I only have mtgcardsmith so I don’t have enough room to fix everything, unfortunately.)