Acharya Nimbarka was incarnation of Lord Narayan's Sudarshan chakra ,as lord himself sent Sudarshan Chakrs to incarnate the earth to restore Dharma by request of devotees.
The story is compiled from book on Hinduism Today site , where an interesting and detail story of Nimbarkacharya is given.
The story of Nimbarkacharya Nimbarkacharya aka (incarnation) of Sudarshan Chakra happend in the Braj region.
The book is presenting excerpts from the ancient hagiography "Nimbarka Vikranti" renderd into english by "Brahmachari Vrajavihari Charan"
The Story -
In the Mathura the Main city of Braj region ,Non Hindu rulers built many rich edifices to attract and to convert Hindu people / Sadhus to their religion. People Sadhus stopped following ancient vedic practices. Many of the Ashramas were even in danger.
So a group of Sadhus decided to find solution to this problem. And they went to the last remaning Ashrama of Rishi Aruni ,who was descendents of great Bhrigu Rishi. They told Aruni Rishi about the situation that "Mathura ,Braj & Vrindavan is no longer safe for devotte's. As a solution Aruna Rishi encouraged them to perform "Gopal Yajna" in his ashrama as lord promised that "whenever there is declin of dharma ,he will incarnate to restore it" BG 4.7-8.
In the Vaikuntha Lord Narayana listened to the prayers of Sadhus and
The Lord instructed Shree Sudarshan to incarnate in the Tailanga region ,south of Narmada in the Aruna Rishi's ashram on the southern bank of Godavari river. He will then meet Narad Muni , who would instruct him on the secrets of the scriptures.
And with that knowledge he would have a vision of the supreme Lord in the original eaternal state. And will restore the Dharma.
You must restore Dharma in my lands Badrikavana , Braj ,Vrindavana
,dwaraka. And then lord Sudarshans accepted lord's command
Then Incarnation of Sudarshan Chakra-:
Here at Aruni Rishis ashrama his wife became pregnant. And on full moon of Kartika ,the wife of Aruni Rishi gave birth to a son , thus Sudarshana incarnated on earth. His name was Niyamananda.
The Name NibarkaCharya-:
Lord Narayana instructed and dispatched Brahma to signal Niyamanada about his true form.
Once Niyamananda (Sudarshana) by his powers stopped the sun from seting.
Once a Buddhist monk came to his ashrama and told that he is new to
area and because the sun already setted he came there. Nityananda
offered him the food , but he refused by saying that it's already
dark and sun is no more in the sky. But the Niyamananda told the monk
that it's because of the thick forest the sun is appearing to be
setted. The sun is just behind the trees ,and requested to take the
meal. Delighted by knowing that the monk took the meal. And as he
took his last morsal the sun immediately disappeared as it was already setted 96 min before. But to the monk ,it appeared as though ,it was just the sunset.
The monk was actually Lord Brahma and seeing the powers of Niyamananda ,
Brahma reveled hid true form and said. "Lord Brahma told in soothing
tone to Niyamananda retracting the commands of lord Narayana "O mighty
sudarshana whoes brilliance is equal to thousands of sun's ,revel the
divine path to those who are blinded by the darkness of ignorance. You
must now accomplishe the goal of your incarnation. And because you
have shown me the sun (arka) amidst of neem (Nimba) tree you will be
heteforth be known by the name of "Nimbarka"