Pendulum Effect: If you have no card in your Pendulum Zone: you can target 1 monster in your GY that you can place a Spell Counter on; destroy this card, and if you do, Special Summon that monster, and if you do that, place 1 Spell Counter on it. You can only use this effect of "Mythical Beast Medusa" once per turn.
Monster Effect: Each time a Spell Card is activated, place 1 Spell Counter on this card when that Spell resolves. Once per turn, during the Battle Phase (Quick Effect): You can remove 2 Spell Counters from your field, then target 1 face-up monster on the field; its ATK/DEF become half its current ATK/DEF until the end of this turn.