Akuru is an evil god in Japanese Methodology.
Akuru was a giant fish which used to live in the Anaumi Inland Sea in the Kibi Province (current Okayama Prefecture). The size of the fish was huge enough to drink a nearby boat in one gulp. In one theory, akuru is regarded as the 'akujin' or 'Kibi no Ananowatari no kami' which appear in the 21st year in the Keiko ki (records regarding the time of the Emperor Keiko) in the "Nihonshoki" (Chronicles of Japan), or 'Anato no kami' which appear in the article regarding the Emperor Keiko in the "Kojiki" (The Records of Ancient Matters).
On his way home from subjugation of kumaso (a tribe living in the ancient Kyushu district), Yamato Takeru met this akuru. Yamato Takeru quickly mounted the back of akuru who was rampaging and killed it with his sword. There is another theory that Susano also fought against akuru, but the truth is unknown.