Blood Mefist (YCSW-EN004) [Yu-Gi-Oh! Championship Series Prize Cards]

1 Tuner + 1 or more non-Tuner monsters
Once per turn, during your opponent's Standby Phase: You can inflict 300 damage to your opponent for each card your opponent controls. Each time your opponent Sets a Spell or Trap Card(s): Inflict 300 damage to your opponent.