Once Sage Shilad did a tremendous penance to please lord Shiv with an aspiration to have a son. Lord Shiva appeared before him and asked him to demand any boon he wished. Sage Shilad expressed his desire of having a son who is not born from a physical body and who is proficient in all the scriptural knowledge. Lord Shiva blessed him by saying - "So be it". Sage Shilad then returned to his hermitage and performed a Yagya. From the Yagya Kund appeared a child who possessed four arms and three eyes. Sage Shilad was very pleased to see that child. The birth of the child was celebrated with great fanfare. Lord Shiv and Paarvatee arrived to bless the child. The child was named Nandee as his birth had given immense joy (Aanand) to sage Shilad. Later on, Nandee lived with his father like any other normal child and was brought up with great love and care. He became proficient in all the scriptures within seven years.
Being inspired by Lord Shiv, two Braahman came to sage Shilad and informed him that after one year Nandee may return back. Sage Shilad became extremely sad. Seeing his father in his sorrowful mood, Nandee consoled him and later on went to do penance. His tremendous penance pleased Lord Shiv and Paarvatee and both of them appeared before him. Lord Shiva blessed him and said- "You are just like me, so you will never die". Lord Shiv also gave one of his garlands to him. As soon as Nandee wore that garland he imbibed all the qualities of Lord Shiv. After that Lord Shiv took out some water from his locks of hair and sprinkled on him, as a result of which five rivers came into existence. These five rivers were, later on, came to be known as Pancha-nad. Lord Shiv then made him the leader of all his Gan. Later on, Paarvatee took Nandi under her guidance and considered him just like her own son. Nandee was married to Suyashaa, the daughter of Marut. Ultimately all of them accompanied Lord Shiv to his abode. There was no effect of the words of Braahman.
Nandikeswar is an incarnation of Lord Shiva himself as told in Shiv Puran by sage Soot Jee.