To talk about the changeling mechanic implies talking about creature types. As of Kaldheim, there are 252 subtypes that appear on the type line of creatures, of which 228 are printed on actual creature cards. (The rest of them only show up on tokens or are applied as modifiers to existing permanents; this was the case for Coward before the printing of Craven Hulk, for instance.) The creature type system in use today saw major refinement in conjunction with the release of Lorwyn in October 2007, when the so-called Grand Creature Type Update was enacted.
Lorwyn is also the set that introduced an ability, changeling, which seemingly made creature types meaningless, as cards with that keyword don't just have one or two subtypes—they have all of them. Before being keyworded, the concept had already been used on one card, Mistform Ultimus from Legions, released four years prior.
Legions was a very peculiar all-creature set, the second in a block, Onslaught that would go on record as the very first to explore the tribal theme in a systematic manner. The design of Mistform Ultimus was trying to play into those dynamics by adding a "wildcard" creature belonging to each and every tribe, so that any of them could employ its services. This idea was conceptually sound, but the execution proved far from ideal, since a vanilla four-drop 3/3 doesn't suit a tribal curve that has other options at that spot, and in general doesn't help a tribe perform optimally. The problem was compounded by the fact that it was a rare, reducing even its Limited applications. This dissonance between intent and implementation grandfathered by Mistform Ultimus would later plague most of the first batch of its direct descendants.
Lorwyn block introduced twenty creatures with the changeling mechanic, thirteen in Lorwyn proper and an extra seven in the small follow-up expansion Morningtide. The former set also featured six noncreature changeling cards that made use of the then new (and now obsolete) tribal card type, allowing them to have creature types even while not being creatures. Additionally, Runed Stalactite would grant the equipped creature every subtype without formally using the changeling keyword—it would later be slightly reworked as Amorphous Axe; more famously, Mutavault would make a larger impact as the "changeling manland," a concept recently reprised by Faceless Haven.
Like Mistform Ultimus, the changeling creatures have one type explicitly stated on the type line—entirely for flavor reasons, since the changeling ability clearly establishes their type line should feature every obtainable creature type at all times. The old Mistform Ultimus was an Illusion, while all the changelings from this point onward claimed membership among the Shapeshifter tribe. In keeping with Lorwyn's overall Celtic inspiration, Wizards borrowed the notion of changeling from North European folklore, though only Crib Swap actually references the myth of the faeries stealing a newborn and replacing it with a supernatural doppelgänger.
These first changelings were equally present in all five colors, each of which received two commons, one uncommon, and one rare. Once again, the problem was that these cards squarely fell in one of two typologies: either they are pure Limited fodder (after all, the mechanic's raison d'être was that the tribal theme needed some "glue" in Limited) or they are individually powerful cards that don't particularly care to be placed within a tribal shell.
The latter is the case for three of the rares, which would go on to become the only changeling cards from the block ever to enjoy a significant amount of Constructed play—with the partial exception of spot removal Nameless Inversion, which can get endlessly recurred by Haakon. Specifically, Mirror Entity fits any deck with a go-wide strategy; Taurean Mauler simply grows on its own; and Chameleon Colossus only needs trample in order to thrive. The Entity's predilection—to enter a well-populated battlefield—correlated naturally with the goals of tribal decks, and its ability to turn Wirewood Symbiote into an Elf figured in some infinite loops. Otherwise, decks and players largely ignored their tribal affiliation.
For the longest time, the original twenty changelings were the only help available for Tribal Wars builds featuring tribes that had no other way to reach the required twenty creatures. But the unbalance between colors was a concern. Aside from the three mentioned rares, the most playable changelings were mediocre (French) vanilla creatures such as Avian Changeling or Woodland Changeling. Red didn't even have one of those and had to rely uniquely on Taurean Mauler. Blue and black lacked a worthy rare, as Shapesharer is too situational and costly, while Cairn Wanderer is just plain bad, forcing black to default to pretty terrible filler like Moonglove Changeling, a three-drop that has to pay an activation cost just to get deathtouch.
Twelve years had to pass, but in the end Modern Horizons revitalized the changeling mechanic. It was a welcome return, but the experimental set's new approach to changelings still had its flaws. For one thing, it mostly focuses on white and black. Red is entirely excluded, while green has arguably the worst card of the group in the overcosted Webweaver Changeling, and blue is but one half of the mechanic's first multicolor card, Unsettled Mariner.
But what the Modern Horizons designers successfully recognized is that, in order to appropriately complement a tribal curve, the changelings must lower their cost. You want to drop early creatures that might not look like much but are made better by the linear synergies that come afterward, not vice versa.
Game-Trail Changeling at five doesn't help anyone, because that's when you should drop a heavy hitter, not a French vanilla dude that could perhaps interact with the rest of the tribe. Conversely, the Modern Horizons changelings give every color access to something straightforward like Universal Automaton, and black access to Changeling Outcast, which is especially great with ninjutsu. (Black also finally got a three-drop that doesn't need to pay to have deathtouch.)
White claims the lion's share of these ten new changelings. Impostor of the Sixth Pride is just the mechanic's take on a typical Limited design, but it's still more efficient than any two-drop with changeling printed previously. The double deal of Irregular Cohort is fairly costed at four. Valiant Changeling is the first to interact with its own "tribe," since the presence of another changeling is enough to make it into a pretty attractive 3/3 double striker for two mana. Unsettled Mariner (clearly inspired by H.P. Lovecraft's The Shadow over Innsmouth) introduces a useful ability on an early changeling; now you don't have to run them uniquely as random filler, they can have intrinsic value! The same is true of Graveshifter, a functional reprint of Gravedigger with changeling attached. There's a world of possible nuances between the abysmal power level of Changeling Sentinel and the pinnacle of Mirror Entity.
However, the most distinctive member of this second wave of changelings is definitely Morophon, the Boundless, the first legendary creature bearing the keyword, as well as the odd nonartifact, non-Eldrazi creature with a colorless mana cost, albeit a five-color identity. The latter is the key to Morophon's design: it's a commander for any flavor of tribal deck, providing an anthem and cost reduction to the chosen tribe. Of course, in a Morophon deck, every changeling is at home.
Luckily, we didn't have to wait another decade to get new changelings. Kaldheim revisited the concept for a third time, and with more purpose than ever. Now the changelings have a precise in-universe identity as the masked dwellers of the mysterious, ever-changing Littjara realm. (Not sure why beings that can literally change their faces should bother to wear masks, but the aesthetic is striking.)
They also exist strictly in blue and green, two colors Modern Horizons had neglected and that, when placed together, give birth to proteiform entities like the Simic Combine, so the flavor seems accurate. Green now has value-based two-drops like Masked Vandal and Guardian Gladewalker, which render Woodland Changeling completely obsolete. At three, Realmwalker establishes a link with the tribe the changelings are supporting, offering a crucial card advantage ability along the lines of Vizier of the Menagerie but at an even more efficient mana investment to get it online.
Blue owns the minority of the new cards, but the first mythic in Orvar, the All-Form, which is also the first changeling to suggest a build-around approach. It's the second legendary member of the Shapeshifter subtribe, and it's accompanied by the third, the universal clone Moritte of the Frost (which, among other things, creates an infinite loop with Harald Unites the Elves, since Moritte can be an Elf in the graveyard and a legendary Saga on the battlefield).
All colors get access to a new colorless changeling, the three-drop Bloodline Pretender, which plays similarly to Diamond Knight but for creature types rather than colors. All in all, not a single one out of the ten changelings from Kaldheim exists solely to carry the changeling keyword; they all have additional abilities, some more appealing than others, but none entirely without merit—even in the more Limited-oriented cases like Mistwalker and Littjara Kinseekers.
On top of that, Kaldheim contributes to make the changelings feel like their own tribe (the Shapeshifter tribe, that is) and not just supplements for other tribes. The Bears of Littjara (I guess those are not actually bears, are they?) is tribal enhancement on the second chapter and removal on the third. Absorb Identity can generate lethal alpha strikes. And Maskwood Nexus is mostly a Jenny/Johnny card that plays similarly to Arcane Adaptation but also incorporates a changeling token generator better than the previous attempt, Modern Horizons' Birthing Boughs.
What makes running an all-changeling list intriguing is the opportunity it provides to enable any number of tribe-specific cards, from all the lords and cost reducers to spells that require specific creature types to function, think Mark of the Oni, Dragon Tempest, or Giant's Grasp. Demons, Dragons, and Giants? In our changeling deck, we have the very best of those! They're called Realmwalker, Realmwalker, and Realmwalker, respectively.
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