Wasn't sure if this should be UB or mono-colored. Ultimately though he technically doesn't do anything mono-blue can't, so I went with that even if he is an assassin.

What about this card is an assassin? Right now he's replacing real creatures with "fake" copies. That sounds like an illusionist or, my favourite, some mirrodin/phyrexian artificer.

I'd change him to be named "Poth, Insidious Compleationist (Althought this is a long-ass name)"

Creature type: Artificer (Compare to suture priest who is nothing but a cleric]]]

Abilities are the same except the creature is an artifact in addition to it's other types, and it also says "Another target" so he can't get himself.

"When Poth, Insidious Compleationist (Althought this is a long-ass name) enters the battlefield, it runs out of space in its text box."

I love the flavour here, mind you. It's slowly replacing your opponent's army with copies who stay loyal to their commander until you've got enough infiltrators that they can launch a coup. It's a very blue assassin - needlessly complicated :)

Sadly, I fear it'd play out "I guess I'm playing lots of tokens myself; play this, win" rather than actually doing its flavour. But maybe that's fine. You'll still be able to use it for the last few points of power, and hack, if you have 20 points of power in tokens then you could probably have won.

In multiplayer this alt-win could be an utter beast mind you. But then, there'd be more people trying to kill it, too.

It's very hard to get tokens with CMC. They have to be copies of actual cards, so there's not much of a way to have 25 CMC in play unless you've been using this a bunch

Huh; for some reason I was thinking "power". That does limit the potential a lot.

Well, keep in mind that it can act as a flicker too.

That said, you're right that 25 is probably a bit too high. I don't think lower than 20 would be reasonable though.

Should this say "Exile target nontoken creature or nontoken planeswalker"? Because, thematically, it would seem silly that this assassin would keep killing the same token over and over.

Oh no, I've read that book! The imposter was too faithful to the original and kept switching sides back again, so they had to replace them again.

But that might be more normal :)

I kept wondering what to pair this with. Has the time come for Scornful Egotist? Is it finally Sprouting Phytohydra o'clock? Alas, none of those seem actually that efficient at getting 20cmc onto the field.

Embalm... explicitly zero's the mana cost. It's like they were planning this card :)

There's a few other ways that don't work. I had hopes for Myriad, e.g. Broodbirth Viper which can easily create 20 CMC in one go, but it only works during the combat step :)

Embalm doesn't actually help that much as you're usually playing "regular" prices for the creatures, so it's quite hard to build up 20 CMC worth.

I feel like there must be something that fills up on copied creatures for cheap. Maybe a weaker version of them somehow, e.g. ones that don't usually get an attack step somehow. I think this trigger leaves no room for an "until end of turn" effect even if it's instant. But there might be some other trick, but I can't think of one.

I had indeed forgotten that repeatedly flickering planeswalkers could be a really valuable in itself. Maybe a deck full of planeswalkers is where this is most at home.

First step, make this "exile target non-token...", otherwise yes, it's a repeatable blink (Teferi, Temporal Archmage combo piece)

Secondly, if you want it to be an infiltrator, you have to make it say that it can only target things your opponents control. Otherwise you'll only ever use it on your own stuff as a blink.

Thirdly, if you make it flavourfully an infiltrator, this will never be anything but jank.

Letting your opponent get 20 CMC worth of creatures and planeswalkers on the battlefield? You are dead, so dead.

As a compleationist, this card acts a little like roon of the hidden realm but instead of an unrestrictive cost, you get an alt win condition.

How about making it that they have to have more cmc of token than non-token, as the win-condition?

So if you blink their only creature/planeswalker you win?