The past year has brought to print some of the most powerful Magic sets and cards to ever see the light of day, culminating in Ikoria's companions that have fundamentally changed the way Magic is played. Magic's R&D team has made it clear that anything is on the table, so I didn't bat an eye upon learning that one of Magic's most obscure and confusing mechanics of all time, phasing, is returning in Core Set 2021. While it has long been considered one of the least likely mechanics to return, maybe it was inevitable that it would resurface in 2020.
Debuting in Mirage on just thirteen cards throughout the block—very few with real competitive applications—phasing faded away and is now a relic of history that has been suddenly thrust into the current limited format and potentially Standard, and technically Pioneer and even Modern. Now everyone needs to get up to speed and learn exactly what phasing is, how it works, and how to best use it.
Phasing has a history of multiple rules changes that have altered how cards with phasing interact with other cards and even with themselves, but the basic function has remained constant.
When an ability makes a card phase out, and before the beginning of each untap step for any card with the phasing ability, it and any other cards attached, like auras or equipment, as well as any counters on it, "phase out." For all purposes they become invisible and cease to exist in-game. Phased out creatures can't enter combat or use abilities, nor can they be targeted, and are immune to effects like battlefield sweepers. Any passive abilities won't function, and they don't count towards anything like Affinity or Devotion. A phased out card retains its status, which means it stays tapped or untapped, flipped or unflipped, and face-up or face-down. If it's controlled by a player that isn't its original owner, like in the case of something stolen by Agent of Treachery, phasing out won't change this, but Threaten effects will end at the end of turn. This and any other temporary effects, like a pump from Giant Growth, will end at end of turn when phased out, but if an effect were to phase them back in before the end of turn, they will still retain it until end of turn.
At the following untap step, that player's phased out cards re-enter existence by "phasing in." Creatures that phase in are unaffected by summoning sickness.
Originally the rules designated a special zone for phased out cards, and phasing out would trigger leaves-the-battlefield effects (while curiously not triggering enters-the-battlefield effects), but an update to the rules in 2010 eliminated this zone, and they now simply change status to "phased out", and they technically never change zones through the process. This significantly changed its applications and even destroyed the function of Ertai's Familiar, but it was a small cost to greatly simplify phasing for its original intent, which is to preserve the complete state of a card as an alternative to the typical exile effect like Oblivion Ring that removes anything attached to a card and resets it when it re-enters the battlefield.
Even after the removal of the phased out zone, phasing still destroyed tokens, but Teferi's Protection, the first non-silver bordered (Old Fogey) phasing card since Mirage block, brought another change that state tokens are now treated the same as any other card, and survive their phasing journey completely intact.
Phasing lends itself to some surprisingly versatile applications. The first phasing cards were designed to use the ability in all sorts of weird ways. Its most common application was as a drawback, essentially halving the effectiveness of any card featuring it. In the case of Teferi's Isle, the land produces two mana but can only be used every other turn, so it ends up balancing out to a normal-powered but wonky land. More typical examples are creatures like Sandbar Crocodile and Breezekeeper, which at the time were apparently considered far too mana efficient without a drawback. Having access to a creature only half the time is a serious drawback indeed, and without the stats or abilities to make up for it, these are among some of the worst Magic cards ever printed. Any new cards with the ability primarily as a drawback will have to offer some very serious upside to be playable.
Other cards use phasing in more creative ways, like Teferi's Imp that acts like a split loot effect. Shimmering Efreet reveals the other side of phasing, using it as a drawback to bestow on opposing cards, as featured on Teferi's Curse and Teferi, Master of Time. Turning phasing against the opponent has obvious applications as a sort of lightweight removal or bounce spell, most closely resembling detain. While this is far from a true removal spell or even bounce, it does have some unique benefits, namely that it plays well against creatures with enter-the-battlefield effects that would otherwise be poor to bounce or risky to hit with a card like Banishing Light.
Head of Magic R&D Mark Rosewater confirmed on his blog that the return of phasing was an experiment for dealing specifically with these creatures, which have become an ever-present threat.
A great example is Uro, Titan of Nature's Wrath, which goes beyond being good against temporary exile effects with escape adding another layer of protection against traditional removal spells. It makes phasing a uniquely useful solution only surpassed by true exile effects like Path to Exile.
When phasing out can be applied at instant speed, like on Sapphire Charm or Teferi, Master of Time's ability, it has the further upside of being able to actually interfere with opposing spells. The simple example is phasing out an opposing creature in response to an effect like a pump spell or aura, rendering it no longer a legal target and effectively countering the spell.
R&D was careful to specify that Teferi, Master of Time's phasing ability can only target opposing creatures, because this concept can be flipped around and turned on one's own creature to protect them from removal spells. The ability to phase out of harm's way proved incredibly useful on Rainbow Efreet, the best card with phasing, successful as an unkillable finisher for blue control decks that could just as easily dash around the deck's Nevinyrral's Disk as it could opposing removal. It was even more impressive when damage went on the stack, which allowed it to stack combat damage before phasing out safely away from any damage it would receive.
Reality Ripple opening up the phasing ability to more permanents has led to some even more interesting uses, like for saving Phyrexian Dreadnought in response to its trigger and essentially Stifle the effect. If cards like Reality Ripple that allow this sort of versatility see print, I'd look to them as the potentially most useful with the mechanic, with applications that skirt the line between counterspell and removal spell with the potential to unlock any number of synergies. A card like Rainbow Efreet that can protect itself would have very strong competitive prospects.
Teferi, Master of Time's passive ability goes farther than offering the ability to use abilities on either turn to being used on both turns. An extra looting each cycle helps build loyalty for more phasing triggers, or to more quickly reach the game-breaking ultimate. The reality is that a couple looting effects and the tenuous removal of phasing, which is relatively expensive to use, and a costly ultimate, don't add up to make what looks like a slam-dunk playable, especially not compared to some of the more broken printings recently. I wouldn't be surprised if Teferi, Master of Time exceeds my expectations, but I am holding out for even better phasing cards if the mechanic is to make a big impact in constructed. Still, phasing is back, and Teferi, Master of Time is likely just the beginning. It might be only a matter of time before phasing goes from an obscure ability into an evergreen effect.