Kroni is the primal manifestation of evil in the Ayyavazhi mythology, he can take on many different forms such as Ravana and Duryodhana, it is said each form Kroni takes is unique to a specific era of time.
In order to combat Kroni the spirit of ultimate good, known as Mayon, creates its own avatars to fight against Kroni's chosen forms in each era.
Kroni is similiar to Kali but is also, obviously, an entity that is comparable to Satan in Jeudo-Christain beliefs.
Kroni in the Akilattirattu Ammanai
According to the Akilattirattu Ammanai, the Ayyavazhi holy book, Kroni was born in the first of the Eight Yukams (aeons) with multitudinous limbs each the size of a mountain, and was the first evil to be born in the Universe. He had a fire of ravenous hunger in his stomach, and he drank all the waters of the sea to quell it. The water being insufficient, he swallowed Kailayam, the abode of Shiva, and then proceeded to devour the entire Universe. Mayon, residing with Shiva in Kailayam, escaped promptly and undertook a Tavam to receive a boon from Shiva in order to destroy Kroni. Shiva granted the boon, but made Mayon aware of the necessity of appearing in different forms for the successive six yukams in order to destroy the six fragments of Kroni. Being aware of the mission, Mayon sliced Kroni into six fragments and saved the Universe. With that event the first Yukam came to an end.