Disclaimer: Yuugiou (Yu-Gi-Oh!, Yu-Gi-Oh! Duel Monsters) is the property of Takahashi Kazuki and respective publishers, including Shueisha, Studio Gallop, Nihon Ad Systems, Toei Animation, 4Kids, and others. I would never claim to own it or attempt to make money off of it- just think of this as a written, free doujinshi and hopefully we'll get along fine ;)
Notes: Part of my Mukashi Tales series, a collection of short stories exploring the characters and events of the original AE.
Fair Exchange
He may not be a kind one, but he is a god. He will spread his influence over the world, not just the creeping dark corners but over the land and sky, and he will accept their fearful offerings and give them favors in exchange.
Already his servants are benefitting. Would the Thief King have made it this far without his guidance, subtly manipulating and keeping him from the exhaustion and death which might have otherwise resulted? The Thief King is reckless, spilling his life into his quest, and he needs the help. So when his hands touched the receptacle, touched the darkness and false gold of the Sennen Ring, Zorc slid into him like a mist and set about giving him what he wants.
In exchange, Zorc will receive what he wants. It's a good arrangement.
The old priest wants to raise his son up, see him at the highest position in the land, his opportunities not stolen from him by his father but given to him by his father. Zorc can use this as well.
It's a great change from that damned saintly priest, who bore the darkness of the Sennen Ring but never faltered, looking with eyes only turned to his duty and those who fell within it. He was useless- there was nothing within him to be turned to Zorc's advantage, so Zorc caused him pain for amusement and left it at that.
If the priests take the Ring back from the Thief King, then that little student will inherit it… Her master matters more to her than her duty. He could use that. She will have what she wants, and in exchange he will have what he wants.
Let it never be said that the Dark God Zorc does not pay his subjects well.
AN:
Basically this is my theory on how Zorc's possession in the anime worked. And I'm not saying that he ever possessed Mana, just that he probably would have tried if given the opportunity.