Mirror Entity

Mana cost: {2}{W}
Type: Creature — Shapeshifter
Power / toughness: 1 / 1
Set: Murders at Karlov Manor Commander #75 (rare)
Price: $2.23 USD

Changeling (This card is every creature type.)
{X}: Until end of turn, creatures you control have base power and toughness X/X and gain all creature types.

Mirror Entity has 14 printings, first in Lorwyn (2007). EDHREC ranks it #1,009 among Commander cards. It appears most in Rin and Seri, Inseparable decks (13,903 on EDHREC). 10 public decks on TCG run it. It has 3 official rulings.

Legality

commander: legal
legacy: legal
vintage: legal
pauper: not legal
modern: legal
pioneer: not legal
standard: not legal
brawl: not legal

Public decks that run Mirror Entity

Bangarang!, led by Derevi, Empyrial Tactician
Deadly Disguise - Murders at Karlov Manor Commander, led by Kaust, Eyes of the Glade
Evasive Maneuvers - Commander 2013, led by Derevi, Empyrial Tactician
Evasive Maneuvers - Commander Anthology, led by Derevi, Empyrial Tactician
Everyone's Invited - Secret Lair Drop - WUBRG EDH Precon Decklist, led by Morophon, the Boundless
Most Wanted - Outlaws of Thunder Junction Commander, led by Olivia, Opulent Outlaw
Open Hostility - Commander 2016, led by Saskia the Unyielding
Party Time - Commander Legends: Battle for Baldur's Gate, led by Nalia de'Arnise
Raining Cats and Dogs - Secret Lair Drop, led by Rin and Seri, Inseparable
Spirit Squadron - Crimson Vow Commander, led by Millicent, Restless Revenant

Commanders that run Mirror Entity most

Deck counts from EDHREC.

Rin and Seri, Inseparable: 13,903 decks
Voja, Jaws of the Conclave: 11,720 decks
Atla Palani, Nest Tender: 7,848 decks
Urtet, Remnant of Memnarch: 5,486 decks
Morophon, the Boundless: 4,468 decks
Millicent, Restless Revenant: 4,366 decks
The Destined Warrior: 3,062 decks
King of the Oathbreakers: 2,423 decks

Often played with Mirror Entity

Cards that show up in the same decks on EDHREC.

Rulings

2021-03-19: Changeling applies in all zones, not just the battlefield.
2021-03-19: Mirror Entity's ability overwrites any effects that previously set a creature's base power and/or toughness. Any existing effects or counters that raise or lower a creature's power and/or toughness continue to apply to the creature's newly-set power and toughness.
2021-03-19: Activating the ability with X = 0 will cause all creatures you control to become 0/0 and be put into the graveyard.