Lord Shiva appeared as Sri Shankracharya.
He re-established the supreme authenticity and decorum of the Vedas, and founded the philosophy of ‘brahmakaran-vad’ (Brahm as Ultimate cause).
In later ages, the Vaishnav stalwarts built the philosophy of bhakti upon this same foundation stone.
Source: Dasavatar by Srila Bhakti Ballabh Tirtha Goswami Maharaj