The defining event of Ashoka’s rule was his campaign against Kalinga (present-day Odisha) in the eighth year of his reign.  

• This is the only recorded military expedition of the Mauryas. The number of those killed in battle, those who died subsequently, and those deported ran into tens of thousands.  
• The campaign had probably been more ferocious and brutal than usual because this was a punitive war against Kalinga, which had broken away from the Magadha Empire (the Hathigumpha inscription speaks of Kalinga as a part of the Nanda Empire). 
• Hathigumpha inscription at Udayagiri caves is the main source of information about Kalinga ruler Kharavela. This inscription, consisting of seventeen lines has been incised in deep cut Brahmi script. 

Source: Tamil Nadu state board