Story behind why Kali is given as Prasadham
A devotee named Sendanar had the habit of eating food after offering it to Lord and then distribute among other devotees. Only the leftover he will eat. On the thiruvadhirai day , due to constant bad weather he couldn’t get any essential items for preparing a proper meal. He had no other option but to mix water with rice flour and prepare a mashed paste. Lord Shiva understood his predicament. He wanted the people to know how sincere his devotee was to him. He disguised himself as a Shiva devotee and enjoyed what was given to him as food. And as he stood for a while in the Nataraja Sanctrum, The Lord showered the Kali all over the premises, in recognition of his deep devotion. Since that day Kali is the special offering to Lord Nataraja on Margazhi.