I didn't have a problem with other cards, but this specific one somehow doesn't work (even after logging out and logging in again).

@Izaac- I wasn't aware of/ considering voluntarily revealing the card. In all honesty my thoughts were ml graveyard and top-of-library. I guess any non-hidden exile would apply too, which I didn't consider. Yeah, voluntarily revealing would male this much stronger. I also definitely didn't consider the complications of whether or not it can be destroyed, exiled, etc. in such a state.

3.12 Hidden Information

"[...] However, players may choose to reveal their hands or any other hidden information available only to them, unless specifically prohibited by the rules."

At first, I thought rule 3.12 would make this card unplayable. But that 'unless specifically prohibited by the rules' is a pretty useful bit. Technically, this could be rolled into the keyword Incomprehensible. It would require some really weird wording in the rulebook, because it's pretty much telling players that revealing the card is verboten, unless any spell or ability asks them to reveal the card for any reason. But I don't think you'd need to put that information in the reminder text, since most players would just presume that must be the case.

But yeah, I would say that the combo with Platinum Angel, Lich, etc., takes an already confusing card and dials it up to 11 in complexity.

So if I have this card in my hand all I need to do is reveal it and it is essentially on the battlefield? Wait, another step back.

This creature can attack and block as if it were on the battlefield but... is that all? It can't be targeted? Destroyed? So if you reveal this from anywhere, as long as it's revealed it's invincible. And if this were legendary I think you could have multiple since none of them are ACTUALLY on the battlefield.

Also, there very rarely exists a player with less than 0 life right? If a player loses they leave and are no longer a player right?

Two landfall triggers. Double ramp with something like Dryad, Prismatic omen or Chromatic Lantern.

The thing is that with any of the cards the card is super strong so I wouldn't want to make it any stronger. I definitely feel this would have to be printed in a landfall set to make sense.

Whoops, I meant to type Dominion instead of Domain.
Yeah, I was messing around with having cards start outside of your main deck again. This definitely was a bumble. The requirement makes this so fringe that I can't think of why a deck would willingly sideboard this for any reason other than fringe on-theme but lol-this-card-exists reasons if someone ran a deck with Phyrexian Unlife or Platinum Angel.

Ha! Reminds me of an old card of mine that I coincidentally came across today: Claim of Sedris.

I like you accounting for converted mana cost. My card gets to cheat casting cost, but is single use. The repeatable version also makes for a nice legendary.

As of right now the intent of this card seems to be to exist in your sideboard without revealing it. I think cards that can end up influencing the game should be public knowledge like companions or conspiracies, or at least their existence should be established e. g. conspiracies with hidden agenda.

This is in many ways a less up-front take on the companion concept.

The requirement is so outlandish that this almost doesn't matter. Even with incomprehensible a workable concept this card is extremely niche and hence a hard sell of the idea.

On a technical note: That last ability is a triggered ability, so this doesn't work at all unless the card is already revealed. Abilities cannot trigger if on hidden cards. Maybe you want this to be a static ability defining a special action, but then you wouldn't have this target.

Domain is already the name of a mechanic that has been colloquially used for a while and then turned into an ability word. That's well-established terminology that you shouldn't try to reclaim, but I imagine there are plenty of suitable synonyms.

Lastly... is domain supposed to work from the sideboard? With the other issues about whether this should be revealed or not this is the weirdest.

It also appears this card having a mana cost is a red herring; well until someone bounces this to your hand, I suppose.

Is sacrificing this card once you get it onto the battlefield the best play?

That's quite an awkward cost to do nothing by itself.

Should I also mention Teferi's Isle? I have that card in one of my commander decks, and it's pretty good. It's also tapped and exhausted before I get to play it, no matter what round it enters the battlefield. And I don't even get to use it half the time. But I don't need all my mana every turn.

Admittedly, I started running it because it's in my Obliterate deck. The phasing is intended to sneak past global land destruction effects. But to be honest, it's far more common for me to use the Teferi's Isle to put enough mana in my pool to cast Obliterate. I found the deck likes using it for ramp, more than for tricks.