Master Tonberry is a fiend from Final Fantasy X, found in the Omega Ruins. Like most tonberries throughout the series, it is virtually identical to its little brother, albeit stronger, with a Karma spell dealing 10x the damage.
When hunting for Master Tonberry, it can be beneficial to go to the second level of the Omega Ruins after Omega Weapon is defeated, as he's among the 7 possible formations one can encounter there, compared to 18 on the first level.
Master Tonberry can drop armor with Death Ward or Deathproof. It can also drop weapons with Slowstrike or Deathstrike. Master Tonberry are one the two enemies in the game that yields one of the rarest items in the game, Tetra Elemental, when stolen from. The other one is Seymour Natus.