The second main expansion of Diamond and Pearl Series featuring Generation IV Pokemon and Pokemon LV.X, Mysterious Treasures was the introduction of Pokemon that carry held items, this effect stays active whenever the Pokemon is in play.
Released in August 2007, Mysterious Treasures features 123 cards, 1 secret card and several LV X cards - Lucario LV X #122, Electivire LV X #121, & Magmortar LV X #123.
Pokedex
Diamond & Pearl
POP Series 6 
