Originally Posted by
ObsidianFire
The only souls we've seen posses bodies are souls with the Echo or the Resonance. Matoya, as far as we know, has neither. Body possession also seems to be linked to the Ascians and has never been shown to be something good for people to do. So until Patch 4.56 comes, I'm putting that view of the situation lower down on my list of explanations for the scene.
As far as Y'shtola talking like Matoya goes... Matoya was Y'shtola's mentor and it seems Matoya views Y'shtola as her protege at the very least, maybe even as her daughter. Y'shtola herself makes fun of herself turning into Matoya in Patch 4.4. So it's very probable that Y'shtola picked up on a lot of Matoya's mannerisms over the years. I could easily see something happening to Matoya while she tries to get the rest of the Scions back from being Called and Y'shtola deciding to take up her beloved mentor's legacy in her place.