Random generator gave me Lady Zhurong, Warrior Queen and Island. Island color shifts me blue, which has roughly no effect. Horsemanship, however, a non-supported ability needed changing. This is me simultaneously thumbing my nose at Horsemanship, while opening up some strange interaction. 4/3 seems perfect for this, being likely to register a block with potential profit.

As a quick aside, 36 creatures mention Horsemanship (though a few, I'd imagine, are tacked onto sorceries), while 82 cards mention 'unblockable' (though, many of those cards, like Curtain of Light don't count either). Which raises the problem "So... what can this be blocked by anyways?" Technically, it can be blocked by Covert Operative, but not by Anaconda, even if you control a swamp. That's because Swampwalk requires a defending player for it to be unblockable.

At first, I didn't include the line about Knight of the Periphery being blockable by cards named Knight of the Periphery because I liked how it messed with your mind when you thought about it too hard. But pointing the card at itself does help people understand what is a valid blocker and what isn't... being responsible is no fun sometimes.

Also, I want this card to be common, but I don't want to charge for it, or change its p/t. I may have to, though...

What if I control Anaconda and Swamp? Since I, the defending player, control a Swamp, it's unblockable, therefore can block the Knight, correct? It probably isn't supposed to be thought of that way.

Hmm. I think the answer jmg is hoping for is that no, technically, Anaconda isn't "unblockable" if you control a swamp; it just "can't be blocked".

jmg is on shaky ground here, though, because "unblockable" (like "indestructible") isn't actually a keyword ability. The official line is that it's just an English word with its normal English meaning. But that might mean it's rather dubious to have anything ask about unblockable creatures, because Anaconda, Progenitus, Wind Drake, and even Grizzly Bears can be "unblockable" (in the English-word meaning) in the right circumstances (for the Bears I'm thinking "when defending player has no creatures"). So I'm not sure if this card works or not.

Well, if you're gonna go that route - any non-attacking creature is unblockable (it's not a valid target to choose to block, after all)

True, the ground is shaky. There's a bit of asking the game how it feels about the current game state. "Hey, game! Are my creatures unblockable right now? Oh, also, can you think of a scenario in which a spell that can't be countered can be countered anyways? Thanks. While you're at it, how do you feel about creatures that could get first strike?"

But I'm pretty sure the vast majority of people can parse this, whether or not the rules can.

I admit, dude1818, the idea that landwalkers could, in theory, be unblockable during your own declare blockers step did not occur to me. So I hit the book. This is what I found:

700.5. If an attacking creature is unblockable, no creature can legally block it.

Huh.
Um.
I... don't know? It may be possible that you can't have an unblockable creature unless it is attacking, but if we were to believe that, then Covert Operative would be lying to us when he wasn't attacking. That's weird. The rule, honestly is kind of unclear on the subject. Let's just say that I talked to the rules team and got them to update that rule? After all, we can look at a card and say "such and such a card is indestructible" even if it is a characteristic and not an ability. If there was a card that said "Indestructible creatures can't block", no one would be confused about what's going on... they'd just think it was weird. I assume unblockable works the same way... it just isn't currently spelled out.

Well, if it's just creatures with the "Not quite a keyword but yes, it really is" unblockable that can block it, then that's fine and simple and obvious. And dull.

If you also want 'landwalkers that see land, and flight if you have no flyers (and would it matter if they were untapped) and ...' then you're in for madness of rules fixing, I fear.

I'm not even sure if "indestructible creatures can't block" would work, actually, for much the same reasons. I'm not sure that's a property of a permanent the game can get at.

Me neither. It's kind of hard to prove or disprove, too, since unblockable and indestructible are in a class by themselves. I'm sure that for every argument I could make, there would be a plausible, and equally shaky, counter argument.

I decided to ask the group I know who're most intimate with the subtle, grubby, finicky details of the Comp Rules: the Rules Theory & Templating board on wizards.com. The thread is here. So far, most people don't think this wording works.

Ha! I've spawned whole arguments!

Unfortunately, I can't currently add to the madness going on over there, which seems like a much better venue to argue your head off. I've been having troubles accessing that account. Password is lost in the Aether. I'll have to work on that tonight.