Muzurhythm the String Djinn

Level 3 WIND/Fiend Xyz Effect
1500 ATK / 1000 DEF

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.

Rulings – Yugipedia

Printings:

Starter Deck: Xyz Symphony: YS12-EN040 (Super Rare) - 2012-04-13
Star Pack 2014: SP14-EN028 (Common) - 2014-02-21
Star Pack 2014: SP14-EN028 (Starfoil Rare) - 2014-02-21

Prices:

Cardmarket: 0.05 €
TCGplayer: $ 0.28