Ragu O Ragula, also known as Ragu Ragula or Ragu O Ragla, is a recurring optional boss that has appeared in every game in the series since the inaugural game. Known as "The King of the Monsters" and "The Master of the Beginning and the End," it is notably one of the most powerful and dangerous bosses throughout the games, usually requiring certain processes in order to get to it, such as reaching the furthest depths of the Abyss in Wild Arms, its remake, and Wild Arms 3, or gaining the four seals necessary in Wild Arms 4 and Wild Arms 5. It is also known for surrendering the Sheriff Star accessory when defeated, which boosts all of a character's stats when worn. This tradition was not carried down into Wild Arms 5, however, in which it would give out an accessory called The Omega instead.
Ragu O Ragula's signature attack, featured in each of its appearances from Wild Arms 2 onward, is "One Trillion Degrees." (In the original Wild Arms, it used a signature attack called "Volcanic Bomb.") This is a very powerful fire-based attack that causes massive damage to the entire party, and is capable of wiping out everyone if they are not well prepared.
Where Ragu O Ragula is located tends to vary between the games, though in most cases it's found in the Abyss, an extremely long and deep dungeon where it rests at the very bottom. In Wild Arms 2, it can be found in the Aguel Mine Shaft in Holst, and in Wild Arms 4, it was in the 19th floor of Etemenanki.
Starting in Wild Arms 2 and following in its subsequent appearances in each game, Ragu O Ragula would have its own special music as its boss theme, which is different in each game.