Creature — Phyrexian Shapeshifter
0/0
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→ Infinite death triggers, Infinite ETB, Infinite LTB
→ Infinite +1/+1 counters on a creature, Infinite death triggers, Infinite ETB
→ Infinite death triggers, Infinite ETB, Infinite LTB
→ Infinite death triggers, Infinite ETB, Infinite LTB
→ Infinite +1/+1 counters on a creature, Infinite death triggers, Infinite ETB
→ Infinite colorless mana, Infinite death triggers, Infinite ETB
Treat Activated Sleeper as though it were the chosen card entering the battlefield, except it's also a Phyrexian. Any "As [this creature] enters the battlefield," "[This creature] enters the battlefield with," and "When [this creature] enters the battlefield" abilities of the chosen card will work.
If the chosen creature card has 
in its mana cost, X is considered to be zero.
You don't have to choose a card to copy. If you don't, Activated Sleeper enters the battlefield as a 0/0 creature and is probably put into your graveyard immediately, unless something is increasing its toughness to keep it alive.