Damien took hold of Duke Horacio's existence and maintained it.
He healed the man's body, reconnected his soul, and with those memories he gained, reconstructed his spiritual world.
However, he made changes as he saw fit.
He altered Duke Horacio's confused soul and made it believe it was loyal to Void Palace.
Duke Horacio's new memories now included a scene where he defected from the Foreign Races and Damien took away the Dark God's control over him.
That was partially correct, since the latter half did happen, but obviously, Duke Horacio never surrendered on his own.
Damien wanted to see how much control over Existence he truly had.
It was an incredibly broad concept, so there was no real way for Damien to understand his limits. If he was able to imagine it, he could likely do it with Existence, but he'd never know for certain unless he actually tried.
