When Legacy of the Valiant becomes tournament legal in the TCG on the 24th, Bujin, Gravekeepers and Noble Knights are going to get way stronger, and they're not exactly irrelevant right now as it is. They're more threatening right now on a local level, but all three have reportedly seen Regional Qualifier Top 8's so far this format, so it's not a stretch to think that all three decks will become more popular with the new power boost. Even beyond those decks, Constellars and Evilswarm are a very real threat for anyone unprepared for those matchups;Spellbooks are still powerful and popular; Fire Kings have been gaining momentum; and there are lots of smaller niche decks that people love to play no matter what a given format looks like. Blackwings, Gadgets, Evols, Madolche, Infernity, Dark World, Harpies, Hunders, Inzektors and a ton of others come to mind.
With so many different threats all competing at the same time, we're in that nerve-racking window where you really need to cram as much as you can into your limited Side Deck space. With just fifteen cards to address all the competitive themes seeing play Side Deck space is at an incredible premium, but there's a particularly strong choice that's worth discussing: Light-Imprisoning Mirror. I'm actually choosing to bend the rules a little and piggyback another card onto the topic, since it makes a lot of sense to talk about the two together, so Shadow-Imprisoning Mirror's coming along for the ride.Balanced Balance
Because of that, you can use the activation of the Mirrors as a 1-for-1 or +1 to negate a card for free and then instantly slow any momentum your opponent had going. Shifting the tempo of the duel to a more comfortable pace can buy you the time to make your own plays on your own terms, often leaving your opponent in a bad position since they were just trying to commit to a play that didn't work out.
Some cards have deeper strength that isn't really apparent on a surface level, and the Mirrors fall into that category… in more ways than one. Obviously the psychological impact of creating a slower pace of play through scarecrow-tactics is huge, but beyond that you have to remember, we're playing in a format where Giant Trunade and Heavy Storm are both Forbidden. Mystical Space Typhoon's unlimited and generally the most convenient form of spell and trap removal, though Dust Tornado and Night Beam are seeing a fair amount of Side Deck play in Regional Top 8 lists too. Without globally available mass-removal, setting several traps at once or playing out lots of continuous spells next to each other isn't as scary as it once was; you can hold onto your Mystical Space Typhoons for a time when it's actually necessary. The Mirrors take advantage of that just by being good at what they're designed to do, since there's a giant bullseye on each of them the moment either card's activated.
The thing is, if you Side Deck one of the Mirrors you're probably using multiple copies, and the fact that you're essentially forcing your opponent to hold back Typhoons means you're actively strong-arming the situation into your favor. The only two outcomes are that your opponent uses their MST's on your Mirrors and the rest of your backrow's then unchecked, or you lose some other backrow card to Typhoon and your Mirrors are totally safe to continue negating key cards indefinitely. Black Rose Dragon, Dust Tornado and cards like theme definitely do come into play in that scenario, but for the majority of duelists right now, Mystical Space Typhoon's essentially the only means of conveniently removing backrow cards from the equation.
There are a lot of cards that can counter that example, like Harpies' Hunting Ground, Atlantean Marksman, Brotherhood of the Fire Fist - Gorilla and a few other themed options, but you're probably never going to Side Deck the Mirrors against Wind, Water or Fire decks anyways, so in this format it generally works out.
Despite the simplicity of Light-Imprisoning Mirror and Shadow-Imprisoning Mirror, I've seen a lot of people asking questions about what they can and can't negate. It's not directly related, but something along those lines came up when I was at a recent Regional in Seattle: in a game beside me, one duelist tried to use Debunk to negate a Mezuki that would have won his opponent the game. The duelist who'd activated Mezuki argued that it activated in the removed zone, since that's where it was after paying the cost.
A judge was called and he explained that Debunk can negate Mezuki and why, but the fact that the situation came up at all ties into the same kind of surrounding the Mirrors. One question I hear over and over is if Light-Imprisoning Mirror negates Honest, since it lands in the graveyard when it resolves. In fact, if you Google the phrase "Light-Imprisoning Mirror vs" you'll find the number one auto-fill is Honest.
The way that cards work is universal: no matter where the card is after you pay the cost, the effect is treated as activating and resolving in that same location. Mezuki activates in the graveyard, and the effect is treated as resolving in the graveyard even though the card itself is banished first. Honest's effect is treated as activating and resolving in the hand, despite Honest being in the graveyard. It's kind of an interesting mechanic, and it's one that an unfortunately small number of duelists firmly understand. A lot of the confusion came from Skill Drain creating the term "resolving off the field" when referring to cards like Exiled Force and Hand of Nephthys that could effectively dodge Skill Drain's negation. The Mirrors negate anything that activated on the field or in the graveyard, no matter where they end up after that point. Necro Gardna, Bujingi Turtle, Doomcaliber Knight, Thunder King Rai-Oh and any other card that starts out on the field or in the graveyard and then ends up somewhere else before resolution gets shut off.Reflexive Reflection
The Xyz-based strategies among those – Infernity, Evilswarm, Constellars and Thunder Family – all have an awful time trying to put anything onto the field past an active Mirror. Infernity can't search or combo anything, and the other three strategies all lose access to their multiple Normal Summons each turn, reducing your opponent to a Summon-and-pass playstyle if they don't have an immediate answer. Each of those decks has a relatively large backrow, so they can potentially play the grind game and attempt to make something happen, but even if they did it would generally only be a higher ATK beatstick with no effect. The exception to that is Evilswarm Ophion, which would still prevent Level 5 or higher monsters from being Summoned, but the rest are neutered into Normal Monsters.
Bujin can't do anything under Light-Imprisoning Mirror. Bujin Yamato, Bujin Mikazuchi, Bujingi Turtle, Bujingi Quilin, Bujingi Centipede and nearly every Xyz in the Extra Deck are worthless while Mirror's active. Honest and Bujingi Crane are still relevant, but reducing an otherwise strong control deck to a mewling version of the old Little City strategy's nothing a Bujin duelist would be excited about. Inzektors go the same way for the most part; Shadow-Imprisoning Mirror takes away their destruction and they're forced to use Inzektor Sword - Zektkaliber and Inzektor Giga-Mantis to try and overpower the field with raw ATK. They can't destroy your field with Hornet or swarm with Dragonfly, so the only other choice is to play the beatdown game and hold out for MST.
If you can protect the Mirrors from destruction with other card effects, it's usually as good as game against most of those decks. Six Samurai have Musakani Magatama and Legendary Six Samurai - Shi En, and anything that can make Naturia Beast is automatically immune to MST. Gravekeepers could protect Light-Imprisoning Mirror with their new counter trap from Legacy of the Valiant. Dark Bribe and Imperial Custom are still a thing too, no matter how uncommon or unconventional. With other Continuous Traps in the game that aren't too bad right now, even Imperial Custom is worth considering for a very niche setting.And Throw Away The Key
As decks that centralize around Light and Dark monsters get stronger with Legacy of the Valiant, the Mirrors are only going to get stronger and more valuable. They're a compact way to address a wide range of problems with a small number of Side Deck cards, and that makes them perfect for the kind of format we're headed into. If you aren't using them yet and you're playing a deck that could, definitely pick up a few of each; if you can't play them, make sure you're ready to play against them. Either way, they'll definitely have an impact on your metagames sooner or later.