Copy cat

Copy cat, also known as Saheeli Twin or simply Saheeli, is a two-card combo deck formerly played in Standard. Copy cat decks used the interaction of Saheeli Rai and Felidar Guardian to win the game by attacking with an arbitrarily large number of token creatures. The deck came in Jeskai as well as four-color varieties.

Unlike the similar Splinter-Twin combo, which only existed in Standard in 2011 from mid-May (when Deceiver Exarch was printed in New Phyrexia) until the end of September (with the rotation of Splinter Twin in Rise of the Eldrazi), Copy cat stood to overshadow all decks until the rotation of Kaladesh block in September 2018, 20 months after the release of Aether Revolt. R&D had admitted that the combo had been overlooked during development, and as such Felidar Guardian was emergency banned after a half-season of dominance in Standard.

Strategy
When Saheeli Rai's second loyalty ability is used to create a token copy of Felidar Guardian with haste, that token's triggered ability may be used to flicker Saheeli Rai. Saheeli enters the battlefield as a new object with three loyalty counters, so her second loyalty ability may be used once again to create another token copy. This process is repeated an arbitrarily large number of times, creating enough tokens to attack that same turn for lethal combat damage.

Being a two-card combo, the shell in which the combo is supported varied. The two common approaches were in a Jeskai control shell, with Torrential Gearhulk, red and white removal, and blue draw and counters; and the Temur-based energy-blink midrange shell, using Whirler Virtuoso, Rogue Refiner, Oath of Chandra, Oath of Nissa</c>, and a collection of planeswalkers, all of which create board presence and gain advantage with the Felidar Guardian's blink effect.

Four-color
Magic Online user Oliver_Hart finished 5-0 in a competitive Standard League on April 27, 2017, with the following deck, after the release of Amonkhet but just before the banning of Felidar Guardian:

<d title="Four-Color Copy Cat"> Creatures 4 Servant of the Conduit 4 Rogue Refiner 1 Tireless Tracker 3 Whirler Virtuoso 4 Felidar Guardian 2 Glorybringer

Lands 4 Aether Hub 4 Botanical Sanctum 5 Forest 2 Game Trail 1 Island 2 Mountain 1 Plains 2 Spirebluff Canal

Enchantments 4 Oath of Nissa 2 Oath of Chandra

Instants 4 Harnessed Lightning

Sorceries 4 Attune with Aether

Planeswalkers 4 Saheeli Rai 3 Chandra, Torch of Defiance

Sideboard 2 Walking Ballista 2 Authority of the Consuls 1 Dispel 2 Natural State 2 Negate 2 Manglehorn 1 Metallic Rebuke 1 Tireless Tracker 2 Nahiri, the Harbinger</d>

Counter-strategies
A variety of cards can be used to prevent one's opponent from winning with the Copy cat combo:
 * Thalia, Heretic Cathar</c> and Authority of the Consuls</c> prevent the opponent's tokens from attacking by causing them to enter the battlefield tapped.
 * Lost Legacy</c> may be used to remove all copies of either combo piece from the opponent's deck, hand, and graveyard.
 * Commencement of Festivities</c> and other Fog</c>-like effects fizzle the combo for one turn by preventing damage that would be dealt by the tokens.
 * After Saheeli Rai's loyalty ability is activated but before it resolves, Grasp of Darkness</c>, Murder</c>, Stasis Snare</c> and many other instant-speed removal spells may be used to destroy Felidar Guardian, countering the ability upon resolution. Instant-speed burn effects such as Shock</c> and Walking Ballista</c> may also be used to destroy Saheeli Rai during this window. An artifact-supported Unlicensed Disintegration</c> would destroy both cards simultaneously, a major factor of the co-dominance of the deck Mardu Vehicles in this time.