Bacteriophage

When Bacteriophage enters the battlefield, choose a creature subtype. When Phage blocks or becomes blocked by a creature of the chosen subtype, put a 'part' counter on it. During each player's upkeep, put a 'part' counter on each creature they control with a 'part' counter on it. Then, if it has part counters on it equal or greater than its toughness, destroy it and put X token copies of Phage onto the battlefield under your control. (This ability functions wherever Phage is)