Unfortunately, I can't use Catch / Release in this deck as part of the flip side has white in its color identity, which I don't run.

As for Sundial of the Infinite, I refer you to this explanation given on MTGSalvation:

"Act of aggression and its cousins don't set up triggered abilities that return the creatures to their previous controllers; they have a continuous effect that ends at the cleanup step of the current turn.

This is the step that the Sundial will dump you into so you will lose control of the creature on the resolution of the ability on the Sundial."

As for the Blatant Thievery question, you and Keevy are both correct, as I've just discovered. The Blatant Thievery + Radiate combo would only work in a 1v1 scenario, not a multiplayer one.

However, there's tons of other awesome stuff in the deck for Radiate to target as well. xD

I didn't see anyone else mention this, but I am wondering how you skipped Mirari? The same 'copy for 3' combo you get with Uyo and the Prophet wrapped up into one card.

Mirari is a great card. I just don't own one. D: I'll consider putting it in if I can ever get my hands on one.

+1 I like the alternate take on Melek

Cyclonic Rift!!! so much fun in a multiplayer game.

Volition Reins, Mind Control, Control Magic, Corrupted Conscience could be fun, too.

Running basic Clone over Dack's Duplicate?

Dack's Duplicate is on my to-get list. Once I obtain one, it'll go in over the Clone.

If I were you, wanting to copy your opponents deck over and over, I would just drop Black entirely. You're running nearly a third of your mana base for about 5-6 cards including your commander, not that Marchesa isn't a bad commander, but for what you want to do it'd be like running an enchantment based deck and not using Zur the Enchanter.

What you do have is a healthy amount of instant and sorceries, so if I were you I would just drop Marchesa and black entirely and focus just on R/U. For a commander I think your two best choices are Melek, Izzet Paragon for the pseudodraw, and being able to copy your own spells for free is wonderful OR chose Keranos, God of Storms for a commander whose hard to remove and is a threat all on his own because a free lightning bolt every turn go a long way into whittling someone down.

Okay so FatherLiir what would you suggest to replace the black cards in the deck? I'm kind of at a loss for what to think of. (Not including lands; I can take care of those easily.)

I'd rather not include enchant controllers, e.g. Control Magic or Corrupted Conscience, simply because they can just destroy the Aura and they get their creature back. I don't want to control temporarily; I want to control permanently. I guess I can just blink whatever creature I steal, but that just kills the Aura and fills up my GY needlessly.

Well canterlotguardian right off the bat I suggest the artifact that is literally all about taking control of the opponent's deck... By taking control of the opponent! Mindslaver literally lets you beat a player with another player. I'll admit I'm biased for artifacts because of Daretti, Scrap Savant and his Precon deck.

Speaking of the Precon commander decks, I would absolutely take a look at the recent UR deck for some other spells. The legendary creatures it offers, Mizzix of the Izmagnus, Arjun, the Shifting Flame help you deck cycle even if you don't want them for a commander or cast your spells respectively; but the real bonus are the spells Aethersnatch, Synthetic Destiny for something to do with those stolen creatures and spells, Mizzix's Mastery to recast EVERY spell in your graveyard for 8 CMC, it's chalked full of good stuff, even if you don't get the whole deck and just look at the deck list for single spell ideas.

Last, a planeswalker to CONSIDER, not necessarily add. Dack Fayden who is a Bomb in my opinion because straight up stealing a Blightsteel Colossus is fantastic. Either through his -2 OR just targeting it with an unkicked Rite of Replication after using his ultimate.

I do get some of my suggestions aren't good for budget builds and If that's what you were going for, I apologize. But I hope I have helped :)

Budget is a concern insomuch as I won't be dropping for Scalding Tarn any time soon, but other than that I'm good to go. Mindslaver is absolutely amazing, and it's funny because I literally just recommended that card to someone else earlier today. And with that, will come Academy Ruins into the manabase, because I'm a big fan of infinite locks.

As for your other suggestions, I'll touch on a few of them, but rest assured- I'm going to take all of them under consideration. Arjun, the Shifting Flame is nice, but I'm not too big on potentially wheeling away something that I genuinely do need, just because I wanted to cast another spell before that one. Mizzix of the Izmagnus was in the original U/R iteration of the deck, and if I do end up converting back over, he'll likely either be the commander or be in the 99 with Melek, Izzet Paragon at the helm. The rest of the Izzet C15 stuff, I'll be taking a look at as well. I'm waiting until my school refund check drops to get some of the new Commander stuff, but I will definitely be getting that one and maybe the R/W one as well.

And speaking of, how would Blade of Selves fare in this deck? Getting a copy of Diluvian Primordial or Mindleech Mass (if I decide to stay with Grixis) sounds pretty good every turn.

Blade of Selves is a very strong pick, but then I suppose you are going to be putting a target on your back as soon as that comes out and people see what's about to happen. I think of it as a very strong finisher and something you don't want to play too early.

Alright. Now. If, say, I do decide to stay with Grixis, is there anything that should be in here. Praetor's Grasp already comes to mind, but is there anything else you could add on? Maybe Ashiok, Nightmare Weaver?

I like your build a lot! May I ask why you use Wastes? Seems an odd choice, basic colored lands would make more sense... (if I don't overlook something)

Only for Mirrorpool? I'd rather consider the old cycle which can also produce colorless like Shivan Reef. Also: Fixes Mana and lets you loose some life (can be good sometimes when you are on the throne).

Yeah I've been considering removing the Wastes, especially as I can now produce colorless-specific mana with Arcane Lighthouse and Myriad Landscape as well. Those will probably be swapped out for two more Island.

Not budget friendly but Sneak Attack is absolutely insane in Marchesa

In regular Marchesa, absolutely. In this version of Marchesa? Not very much. This isn't your regular combo-centric Marchesa build, where I want to assemble Mike and Trike or some other combo piece and win on the spot. This is a clone/copy/steal deck, that just happens to have Marchesa at the helm because there's really no other viable option besides Sedris, the Traitor King (who, now that I think about it, really should be the general anyways).

nevermind. I found it.

Prophet of kruphix was so broken. It makes sense it was banned...

Seedborn Muse is the next best thing.

Sounds like you've had a lot of dickish roommates.

Um... What are you talking about? @Monsmtg

What I mean is I've been subbed since melek that roommate who always takes your Raman, now it's had so many iterations, I was saying it sound like you have had a lot of dickish roommates, sorry for any confusion