Muzurhythm the String Djinn (YS12-EN040)

2 Level 3 monsters

When a "Djinn" Xyz Monster you control is attacking an opponent's monster, during the Damage Step: You can detach 1 Xyz Material from this card; the attacking monster's ATK is doubled until the End Phase. You can only use this effect of "Muzurhythm the String Djinn" once per turn.

Fiend / Xyz / Effect, Starter Deck: Xyz Symphony