Unbendable Pies Breaks

{Spoiler}Sorcery-speed Burst of Strength on repeat. I'm a little iffy on whether this should cost more. Power-wise I think it can just barely by a scratch fit at 1 mana, but cipher precendent says things would get taxed a little more for having it. It's a mechanic that fit well in most colors though, so good choice.

{Spoiler}Simple but works well. Many of the mechanics chosen for this challenge actually can fit well in their new color, detain is one of the few I wouldn't say does, but this card does have the bit of overcost on top of Mugging to bridge that gap. The flavor seems a little off here though.

{Spoiler}The card seems quite in-line for its cost, and while populate isn't originally a black mechanic at all, black can make tokens and in this case it's replacing a token by sacrificing a different one, a very black-feeling effect. The little extra upside of the channel feels both thematic and great in terms of putting the card's balance just right, not to mention giving you an alternative use if you don't want to sacrifice a token.

{Spoiler}While more thematically white/blue, indeed addendum can work in other colors as well, so it's a good choice here. Double strike is one of those cases where it's a strong enough mechanic that the slightest extra upside pushes it from 1 mana to 2 mana, and you absolutely made it fit. Putting with menace is very clever as well given it's both thematic and there is side synergy in forcing your opponent to give you two targets to hit if they want to block.

{Spoiler}Turning it into a condition to cast the card is an interesting thing to put on a white card, and I honestly have mixed feelings about a white card forcing you to deal damage to an opponent first to use it. On one hand, it keeps you from cheating the card by dealing 0 damage and thus stealing something (even if restricted to a 0 mana thing) for nothing at all. On the other hand, white isn't usually all about making the opponent bleed... I do think it's overall a card that would've been better as a pretty expensive card that you can cut the cost with spectacle than a card that forces you to use spectacle, as the higher cost could also serve as the "toll" you'd need to pay.

Speaking of higher costs, this card definitely should cost more. While the restriction on what you can gain control over is by no means a light one (especially for white given it's not quite the most damaging or evasive color), usually getting an off-color effect like this (if at all allowed) will incur some kind of cost above what would normally be required of a card. This is a restricted Mind Control that doesn't have the vulnerability of being an aura. Honestly I don't think the cost needs to be that much higher, but making this a 4-cost aura (after the spectacle effect) would probably be my suggested fix for the balancing here.

Lastly, I think it's relevant to note that healing an opponent for a large amount of damage is going to be a hard ask for almost any kind of deck. Aggressive decks really don't want to waste the work they put into witling the opponent, and slower decks will have a harder time dishing out great amounts of damage all at once unless they already are in a winning position that makes it less likely for this card to find a use.

{Spoiler}Bloodthirst technically doesn't meet the prompt, as it's from before Return to Ravnica. That being said the reason I put that restriction is because there was a risk the mechanics from before that time would no longer be in line with the current color pie, but Bloodthirst is, so I'm going to let it slide.

I find your card rather interesting because of what it implies about the deck it's put in. In my view, the two most likely homes for a card like this are either an aggressive blue deck, a rarity itself I believe (though I don't much follow competitive decks so I may just not be aware of them), or a more defensive / controlling style that pricks opponents with evasion. Either of these could consistently deal damage letting this card gets the bloodthirst trigger while having a use for its last effect, either as a means of removing a blocker or some other threat.

I'm uncertain as to the balance here. On one hand, cards that force a creature to attack seem to normally be very low stated for their cost. This can come in as as much as a 3/3. There is also the fact that it can untap and become a blocker (I presume it can't force any more stuff to attack though, except if there are extra combat steps involved). Does the power restriction matter enough to let it get this much, especially seeing as most of its uses are likely to be ramming weaker but valuable creatures into your blockers anyway? Only playtest would tell I suppose.

{Spoiler}There wasn't much I could find that would serve as a good comparison to this, Blanchwood Prowler and Tribune of Rot being just about the best ones I could find. Of those Tribune is likely the most apt comparison, attacks and mills then for each creature milled it makes 1/1 which isn't quite the same as getting +1/+1 counters but is as good of an approximation as I could find. I think in a reasonably built deck that runs this card, considering you'd likely mostly be using creatures a 50/50 odds with this card should be an acceptable estimate. As a baseline this card seems about in line with Tribune of Rot, a little bit weaker perhaps since the recasting needs to hit a player specifically. However, there are two things that I think tip the scales here. First, the precedent with Cypher design usually has a big cost tax on the card for having cypher. Second, the fact you get to choose where to put the +1/+1 counters each time means you can more easily put these on valuable targets to grow, or ones that would benefit the most from it. Making 1/1s also has its uses though. In the end, I think this card is undercosted, but only slightly. Surveil as a mechanic integrates really well into green/black too.

{Spoiler}The victory today belongs to melono for a card that really smoothly mixed the new mechanic in, balance, flavor, creativity and synergy, among other nice things in one nice sleek package. @fluffydeathbringers gets the runner-up spot for a card that also gets almost all of that, though I do think Melono's came out a little bit better.