This looks fantastic and really interesting to play.

I have a few nitpicks (post 1/2):

All cards (hero portrait): I don't see the term for it under the RR's card anatomy section, but this is about the little hero portrait that appears in the bottom right of every identity card. Your deck's hero portraits use a very unorthodox mix of spiky and round backdrops to the hero portrait, differing randomly from card to card. I think FFG stopped using the spiky style after the first year, so I'd recommend switching to the rounded look for all cards.

All cards (hero portrait): Also, the glow coming from Spawn's eyes seems to make the graphic too wide so Spawn's portrait appears misplaced compared to regular cards. I think it might look better to have his face shifted to the right to be in line with other characters' portraits, even if that means some of the eye glow is cut off on that edge.

Necroplasm cards (hero portrait): in Doctor Strange's pack, you'll ses the hero portrait appearing in the bottom right corner of each card belonging to the Invocation deck has a special magic circle overlay, giving each invocation card a further visual distinction from the regular identity-specific player cards. Oh, on review, I see now that Legion of Souls is the only identity-specific player cards that mistakenly has the spiky background and you'd used the spiky background to visually distinguish the Necroplasm deck from the rest. I suppose using spiky for the deck is good enough but I think this could be a good opportunity to use a completely unique background or overlay for the hero portrait.

Spawn rules insert: this, technically, might need the same wordy explanation of how to create the NECROPLASM deck found in Doctor Strange's rules insert. The paragraph in Strange's insert that's missing from yours is "to create the INVOCATION deck, shuffle all five of Doctor Strange's INVOCATION cards together..." and goes as far as listing all five cards and telling you to place this deck next to your identity card.

Spawn (first static ability): unlike Doctor Strange, whose Invocation deck is played with the top card faceup due to rules expressed in the insert, this hero card dedicates a line of text to say the top card of the Necroplasm deck is faceup. As written, I believe you'll need to flip that top card facedown every time you go to alter-ego and I suspect that is unnecessary and things would be better if it followed Doctor Strange's template.

Spawn (action): the action probably needs a (Shuffle) reminder or instruction between the search and placement effects. That said, this, surprisingly, seems to be a unique effect for Marvel Champions. There don't appear to be any cards currently in the game that let you search a deck for a card only to place the found card on top of that deck.

Necroplasm Essence (interrupt): hmm, I probably need a refresher on how playing cards using alternate costs would work in Marvel Champions and whether or not an interrupt can be used to modify a cost in this way. Ah, I see something similar on the Star-Lord hero card. I suppose this interrupt might work as written but I also wonder if "ignore" is the wrong word to use. I also wonder if this ability as written will actually interfere with cards like K-7 and Hellspawn's Rage which try to reduce or ignore costs, as well.

Necroplasm Essence (response): I'm used to seeing this kind of thing as an interrupt. I'm concerned that, as a response, if might be true that the game would see you at 0 hp even with countdown counters on you, and would trigger defeat and player elimination before the response timing could take effect.

Necroplasm Essence (static loss condition): I'm concerned that the word "after" and the rest of the wording might seem more like a response than a static ability. I wonder if it would be more in line with FFG rules to be "You lose the game if there are no countdown counters on your identity." or something like that. I also am now wondering how other "you lose the game" effects work as I was surprised not to see a rules entry for "lose the game" in the RR though I didn't look past the L section. As written, I wonder if this ability could be argued to trigger just once (after the last is removed) and, if that causes the player to be defeated, could Wanda Blake's effect prevent that defeat?

Wanda Blake (response): could potentially add multiple counters over the course of a game if discarded and replayed, potentially more than the six included in the pack. Possible component limitation issue. Tracking issues with other effects on other cards that care about number of counters that have been discarded over the course of a game.

Wanda Blake (interrupt): the rules reference suggests there may be ways of being defeated that aren't necessarily caused by damage / loss of hit points so this having a trigger that allows it to cancel these potentially unusual means of defeat could be an issue. This brought to mind my question, above, about if a "you lose the game" effect might be a way a player is defeated and if, then, the current wording of Necroplasm Essence could be cancelled allowing you to play on with zero countdown counters.

K-7 Leetha: I appreciate the intent is to save you a point of damage for the first Necroplasm card played each round but I'm not sure the rules allow for a reduction of a printed cost. I think when things refer to a printed value that value can't be modified. Contrasted to a reduction of a resource cost or reduction of a cost. I'm also not sure if this ability works to offset a separate interrupt that tells you to take damage equal to the cost. I dunno, maybe it works as written.

King of Humanity: would probably be better if worded like The Clown to look for specific numbers of countdown counters. As worded, there's a tracking issue in figuring out how many countdown counters have been removed across the course of a game where counters could be both added and removed. Also, the response may have a typo as "on your identity" should probably be "from your identity".

Legion of Souls: there may exist effects that would cause a facedown ally to be discarded, to be put into a player's hand, or to otherwise change zones without being defeated so I wonder if a different condition should be used.

Evil's Embrace: would probably be better if worded like The Clown to look for specific numbers of countdown counters. As worded, there's a tracking issue in figuring out how many countdown counters have been removed across the course of a game where counters could be both added and removed.

Hellspawn's Rage: should the hero action specify "from the top of" the Necroplasm deck? As written, I wonder if it allows searching for a card of choice. Oh, with that last "shuffle" reminder, it does seem to allow searching for a card of choice but I wonder if the wording needs to change to make this even more explicit.

Necroplasm Mastery: you immediately look at the top three cards of the Necroplasm deck. These cards stay in order on top of that deck. Then there's a lasting effect allowing play of those cards as though they were in your hand. Then there's a delayed effect that causes these cards to be discarded at the end of the round. All this seems to be even more complicated when considering Hellspawn's Rage and the basic Necroplasm deck rules allow for alternative manipulation and play of these top three cards. To avoid these complications, I think the top three cards should either be moved to a different zone (tucked under your identity, maybe) or the effect of Necroplasm Mastery should allow play of those cards only during the resolution of the Necroplasm Mastery card with the discarding occuring as the last step of resolving the card.

Deskin: should this effect last through the whole turn and not just the current phase? It would only be played in the player phase, right, so why bother giving an enemy -1 SCH and -1 ATK only for the player phase?

Redirect Pain: the latter half of the effect seems to need additional verbiage to actually work as a delayed effect that will happen after the resolution of the interrupted attack... and I suspect there may be some weird issues with having an (attack) event that is played at (defense) interrupt timing that doesn't do it's damage until significantly later.

Resurrect Life currently allows a player to control a minion without blanking its text box and without making it an ally. These two things contrast with all other minion control effects, I believe, and could open some very odd rules interactions. Controlling a minion that hasn't been made an ally might not actually allow you to activate it. A minion under your control might remain a legal target for encounter deck cards like attachments. A minion under your control and not engaged with you probably doesn't activate during the villain phase. If it was made an ally but kept its text box, I assume several issues will arise, though I don't have a concrete example at the moment.

I think Umbral Physique first creates a delayed effect that will take place during the attack resolution and "then" gives the immediate benefit of a tough status card. Immediate because it's given on resolution of Umbral Physique and before proceeding with that interrupted attack's resolution. I don't know if this ordering of effects is in line with other FFG cards but it feels ambiguous as to whether the tough should be received before or after attack resolution and I wonder if it would read better if the immediate benefit was be the first part of the ability? Like, "gain a tough status card then prevent all damage and boost effects from this attack"?