"Granmarg the Rock Monarch" is T-Bone's signature card.
In episode 77, T-Bone uses this card during his Duel against Jaden Yuki. He Tribute Summons this card by Tributing "Mine Golem". The effect of this card then activates, allowing T-Bone to destroy "Dormant Volcano". This activates the latter's effect, inflicting 300 damage to Jaden and allowing both players to add a FIRE monster from their Deck to their hand on T-Bone's next Standby Phase. This card then attacks and destroys "Elemental HERO Sparkman". Later "Elemental HERO Tempest" (who was equipped with "Element Sword") attacks this card. Due to the effect of "Element Sword", "Tempest" gains 800 ATK during damage calculation only. "Tempest" then destroys this card. T-Bone then activates the effect of "Guidance to Ore" to summon another copy of this card. Later T-Bone Tributes the second copy in order to Tribute Summon another copy. This activates this card's effect, allowing T-Bone to destroy his Set "Dowsing Point". This activates the latter's effect, allowing T-Bone to add "Mobius the Frost Monarch" from his Deck to his hand. He then activates "Shadow Moon" to send this card, "Zaborg the Thunder Monarch", "Mobius the Frost Monarch", and "Thestalos the Firestorm Monarch" from his field and hand to the Graveyard in order to activate "Blue Moon" and Special Summon "Demiurge Ema".
This monster's attack is called "Buster Rock" in the anime.