In terms of playtests, mostly it acted like "when ~ attacks and isn't blocked, ~ gets +K/+0 until end of turn." It incentivizes trading when possible, and just generally lets deceive creatures punch a bit outside their weight class, kind of like first strike does.

With modern design, it would definitely be a triggered ability just to make digital play better. Nailing the exact trigger condition is a bit awkward though. Options are...

1. Whenever ~ attacks or blocks, you may give ~ +K/-K until end of turn.
2. Whenever blockers are declared, you may give ~ +K/-K until end of turn.

...Option one makes the ability more defensive oriented, because making decisions after blocks is much better than after attacks, which I don't love. The second ability triggers after blockers but has really weird wording.

I'm concerned about a lack of design space unless it is parameterized. I suppose I'll try to do +2/-2 as much as possible and see if I can come up with enough interesting designs to remove the paramter, but I don't like my odds.

Making deceive commons all go to the same toughness seems reasonable.

This is an interesting mechanic, I'd love to know how it played during playtests.

I do feel it could cause decision paralysis with a bunch of deceiving dudes on the board. Maybe you could apply one of many possible simplifications:

1. Make it a triggered ability like bushido.

2. Remove the parameter and set it to a single value everywhere (e.g. +2/-2 always).

3. At least make all commons share the same conceptual effect, like every deceiving common creature becomes an X/1.

Anyway, please share some of your playtest results.

To be clear, it is an activated ability. Its just reminder text.

As to the "on-board trick" of it, it is a really minor one. It is part of a class of abilities that are extremely normal at common. This ability was originally inspired by rootwallas for example.

Granted, I haven't done a ton of playtests, and I'm sure there are some board states that a few decieve effects makes too complicated, but in the playtests I've done, it hasn't felt any more complicated than first strike. It is a thing you have to take into account when making attacks and assigning blocks, but isn't too much of a burden.

I like the mechanic a lot. There's a lot of triggers you can throw on the stack so limited to once per turn is a very smooth ceiling.

Edit: it wouldn't fit your setting but I would call this ability alter

Deceive should be written as an activated ability, and definitely needs a cost associated with it. It's probably also too much on-board trick to have at common in any meaningful amount

"Protection from nonlegendary" seemed too hard to overcome. I like the new "hexproof from" variant more in that sense.

It wouldn't be much of a stretch for this to be mono- (actually it's a stretch for this to be esper). The colors don't do much here. From EDH perspective I can (grudgingly) understand the added colors, but mono- would feel more appropriate. From flavor perspective this would sound reasonable as well since at the moment the card seems akin to Blofeld from Bond series.

The villain of the story. Name to be updated later.

Functional reprint of Ritual of Rejuvenation. Yeah, it's pretty bad.

This could be or cheaper. Even with the instant speed and cantrip life gain as a spell effect doesn't have much mileage.

Edit: after looking at your skeleton naming I can see you're probably aware lol

You might as well add Confident Captain to that list since it also cost , is a 2/2, and gives you Gadgets thought it has higher risk-reward theme regarding getting those Gadgets.