There was a connection made, but I don't think there were any pieces of souls left behind or drawn into the Hall of Echoes after the soul forge.
I don't think Damian was planning that far ahead when he manipulated a dragon knight into attacking the Divine. The Black Ring was winning the war until the other side formed an alliance with the dragon knights, who tipped the balance of power. Having a dragon knight attack the Divine was a convenient way to get revenge and break up the alliance.
Ygerna was able to detect and exploit the player character's mind after the unfinished slayer ritual and Talana's gift of the dragon powers.
The cutscene comments when Ygerna was resurrected state that Ygerna and Damian were working independently of each other, matching the Divine's speech when the dragon knight was imprisoned. The Divine didn't claim they would both be invincible, just Damian.
Damian couldn't kill the dragon knight. He had already asked Patriarch to open a portal to the Hall of Echoes and been refused. The only hope he had to resurrect Ygerna was to let Zandalor's plan progress that far and try to get ahead of the dragon knight at the end.
There will be a Divinity 3 to finish off the story arc with the Divine and Damian. Hopefully there will be another RPG continuing with the dragon knight, as well.
