Nila Madhava is the name of the idol that was in the possession of a tribal king name Viswavasu. Mythology says that, after the tragic death of lord Krishna, his spirit (or bramha) took a shape of a small stone. Viswavasu found this stone and felt its divinity. So he started worshiping it and name it Lord Nila Madhava. This brahma was later fetched by King Indradyumna by Brahmin Vidyapati for the creation of Lord Jagannath's idol. Its been kept inside a small chamber inside the Lords idol. It is believed that its still present inside the Lord's idol and is transferred to the new idol after every 12 years.