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MuOnline - Season 12.2
 

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The entire game takes place in the lost continent of "MU". According
to the plot of the game, the continent of Mu, which was once ruled by
mighty emperors, was caught up in wars between its many poor
rulers. Antonides, one of the rulers, was deceived and accidentally
brought to the continent a demon called "Kundun", as a result the
rulers of the continent were killed, and in the absence of an heir, the
hope of the continent depends on the adventurers who are supposed
to join forces, fight the demon, and restore peace and tranquility to the
continent. Many creatures, objects and places in the game are taken
from Norse mythology, such as Fenrir, Valhalla and Midgard.

Some claim that the plot of the game is based on James
Churchward's book "The Lost Continent". According to the book, Mu is
one of the continents that disappeared under the sea, similar to
Atlantis and Lemuria. However, there is no connection between the
content of the book and the game other than the name of the
continent "MU" which is also used as the name of the game.