Shapeshifter

Artifact Creature    (CMC:6)

As Shapeshifter enters the battlefield, choose a number between 0 and 7. At the beginning of your upkeep, you may choose a number between 0 and 7. Shapeshifter's power is equal to the last chosen number and its toughness is equal to 7 minus that number.

226 ME4 • ENDan Frazier

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Legal in: Legacy,Vintage,Commander

Oracle Text (click to copy):

8/1/2008
The ability that sets its power and toughness is a characteristic-defining ability. This means it applies before any other effects that modify power and/or toughness.
8/1/2008
The ability that lets you choose a new number is a triggered ability that triggers only once each upkeep.