Agartha is a magical firearm, wielded by Albus in the game Castlevania: Order of Ecclesia. The first time Albus used it in the game was when he stole the three pieces of Dominus. He fired a max shot (Acerbatus) at Barlowe, who used a lightning attack to counter it. This gave Albus the opportunity to flee. During the boss fight with him, almost all of his attacks are coming from Agartha.
The enchanted pistol can not only fire normal bullets, but is also capable of firing strong magic projectiles. Aside from the above mentioned Acerbatus, the gun can also fire an Optical Shot (where it concentrates light and dark energy at the tip of the barrel, then shoots the collected energy in a spiral at the target) and Torpor crystals.
It's possible for the wielder of Agartha to use this powers by channeling the energy from a Glyph into the gun. This raises the question if Albus needed it to use moves like the Quadruple Ignis and his warping technique.
Not much is known about Agartha, for example, nobody knows who made it (although it's fairly certain that a member of Ecclesia made it, because it uses the energy from Glyphs to fire enchanted bullets). Furthermore, it's unclear what happened to it when Albus was defeated by Shanoa. It should also be noted that the gun is named after the fictional underground city of Agartha (sometimes Agharta), purportedly located in the hollow core of the Earth. The name comes from the Sanskrit Aryavarsha (the Land of the Noble/Worthy Ones), and this myth is connected with the tales of Shambhalla/Shangri-La. Given this, the name of this weapon fits in well with Ecclesia's apparent goal of creating an ideal and peaceful world free of the threat of Dracula's return; it takes on an ironic tone when one considers the motivations of both Barlowe and Albus.
An interesting note is that two unused voice bits in Order of Ecclesia's Sound Mode involve Albus calling Agartha "Agate," a possible early mistranslation of アガーテ ("Agaate"). The gun's Japanese name is likely a reference to the German opera Der Freischütz (The Marksman), about a sharpshooter who makes a deal with the devil for the love of a woman. The name of the play's heroine, Agathe, is indeed rendered "Agaate" in Japanese. Albus's voice samples also mention not only his Max Shot but an unused "Caspar Shot" attack: apparent references to the play's protagonist and antagonist respectively.