Shapesharer

Mana cost: {1}{U}
Type: Creature — Shapeshifter
Power / toughness: 1 / 1
Set: Lorwyn #85 (rare)
Price: $5.81 USD

Changeling (This card is every creature type.)
{2}{U}: Target Shapeshifter becomes a copy of target creature until your next turn.

Shapesharer has 2 printings, first in Lorwyn (2007). EDHREC ranks it #9,269 among Commander cards. It appears most in Morophon, the Boundless decks (2,672 on EDHREC). 1 public deck on TCG runs it. It has 8 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 Shapesharer

Everyone's Invited - Secret Lair Drop - WUBRG EDH Precon Decklist, led by Morophon, the Boundless

Commanders that run Shapesharer most

Deck counts from EDHREC.

Often played with Shapesharer

Cards that show up in the same decks on EDHREC.

Rulings

2007-10-01: If Shapesharer itself becomes a copy of another creature, it loses both changeling and its activated ability (unless it’s copying another creature with changeling and/or another Shapesharer, of course).
2007-10-01: The copy effect doesn’t wear off until just before your next untap step (even if an effect will cause that untap step to be skipped).
2007-10-01: If the targeted Shapeshifter becomes a copy of a face-down creature, it will become a 2/2 creature with no name, creature type, abilities, mana cost, or color. It will not become face down and thus can’t be turned face up.
2007-10-01: The targeted Shapeshifter copies the printed values of the targeted creature, plus any copy effects that have been applied to it. It won’t copy counters on that creature. It won’t copy effects that have changed the creature’s power, toughness, types, color, and so on.
2007-10-01: This ability can cause a creature to become a copy of itself. This will usually have no visible effect.
2007-10-01: Effects that have already applied to the targeted Shapeshifter will continue to apply to it. For example, if Giant Growth had given it +3/+3 earlier in the turn, then this ability makes it a copy of Grizzly Bears, it will be a 5/5 Grizzly Bears.
2007-10-01: If the targeted Shapeshifter copies a creature that’s copying a creature, it will become whatever the chosen creature is copying.
2017-09-29: If either target becomes an illegal target, the ability will resolve but have no effect.