So we all know priest can get his opponent's cards. Thought steal, mind vision, etc. - there are lots of "thief cards" in the priest class. But with the magic of Drakkari Trickster, Mischief Maker, and Lorewalker Cho (and Seance for even more of them) we can give the opponent our thief cards.
Ok, so now both players have a bunch of thief cards; what's the big deal? Thing is, when WE play thief cards, we get legitimate cards. When our OPPONENT plays them, they just get more thief cards. Control the board. Steal their deck. Fill their hand with useless cards that just give them more useless cards. Somehow, one way or another, this keeps winning me games.
Questionable Cards and Cool Replacements
If you find yourself wanting some more minions you could play, you could consider Chameleos or Madame Lazul. The trouble I've run into with those two is that because they allow more control over which card from your opponent's hand you get, your opponent can use it to copy good cards you've taken from their deck. This deck thrives on being composed of terrible cards - putting in too many good cards just makes it worse.
Unseen Saboteur is optional as well. I consider him in here for a few reasons: 1) He can sometimes mess up enemy combos or clear enemy minions 2) he can make room in the enemy hand for more thief cards or play thief cards like Devour Mind that the opponent is avoiding using and 3) this card is just funny to me.
The best things to keep in starting hand are Drakkari Trickster, Shadow Visions, Seance, and Mischief Maker. Mischief Maker is a fantastic early drop because not only does it let you draw one of your opponent's cards on your next turn, it effectively denies them a draw on theirs. If you're worried about losing the board early, keeping control cards like Forbidden Words and Excavated Evil is valid as well. You should never keep thief spells in your opening hand - even useful ones like Thoughtsteal. You want these cards to stay in the deck as long as possible so your opponent gets more of them, and you want to get control cards out of your deck as quickly as possible so your opponent gets less of them.
Stealing the Win
This deck has no win condition other than "steal your opponent's win condition. This deck may be silly, but it takes thought to play - you must analyse the contents of each player's hand and deck as best as you can and make up your plan as you go. Tricksters and Mischief Makers are essential, make sure to clone them with seance when you can.
Hold cheap thief cards like Cloning Device in you hand for a while if you don't need to play them immediately. This gives more chance for the opponent to get them instead of something useful with Mind Vision, and lets you get a better sense of what you actually need to find with them before using them. The exception is Mind Vision - use these freely, as knowing what your opponent holds is great information.
Your goal at the beginning is to get trickster/seance going as much as possible, but more importantly controlling the board. You need the opponent to throw cards at your removal (and any shades you draw) for the first bit of the game. Once you've gotten a few tricksters off, and have some cards in your hand from the enemy, start trying to turn things around. Your opponent has been playing cards, and you've been playing cards to remove them, so you both are running a bit low - but you've been gaining nice cards from their deck that you can start playing now. If they try to refuel with the thief cards you so kindly gifted them, they'll just get more thief cards. You have card advantage. Now you just need to find the way to use it.