Lord Shiva, the absolute reality and eternally Pure, has taken many incarnations, say the holy texts like Shiva Mahapuran, Kurma Puran. Shiva puran was recited by Lord Shiva himself to Lord Brahma. It was then initiated by Sage Vyasa to his disciples, Sage Shaunak and Sage Suta for the benefit of mankind.
Saturudra samhita of Shiva Purana mentions that omnipresent Shiva had taken countless incarnations in different kalpas or aeons. Shiva had taken avatars as:
Lord Shiva took the first incarnation in the kalpa called shwetalohit. When Brahma was in deep meditation, Lord Shiva manifested as a small child with a white complexion from the divine physique of Brahma. Brahma named him Sadhyojat. Those desiring to go to Rudraloka should take refuge in Sadhyojat form of Shiva.
Lord Shiva took the incarnation of Lord Vamdev during the kalpa called Rakta. He manifested as a small child with red complexion from the body of Brahma. Brahma prayed to him and Vamdev preached about subtle knowledge of religiousness.
Lord Tat Purush
In Peet kalpa, Brahma wore yellow apparel and was in deep meditation. Peet means yellow. Lord Shiva manifested as a child with yellow clothes, yellow turban, sacred thread and garland yellow in colour. He was luminous and bright with halo all around him. He was Lord Tat Purush or Maheshwar and Goddess Gayathri came from him. Brahma chanted Shiva Gayathri and Maheshwar taught Brahma the secrets of yoga. Praying to Maheshwar helps one to get rid of all sins.
In Pradhritta kalpa, when everything was surrounded with darkness, Brahma wanted to commence creation and Shiva as Aghoresh with dark complexion manifested before Brahma. He was wearing black clothes, black crown with a black sacred thread. He blessed Brahma to start creation after pralaya. Terrible sins get absolved through sincere devotion to this lord.
In the Vishwaroopa Kalpa, Lord Ishaan manifested to help Brahma to show the identity of a divine cow that manifested as a result of Brahma`s meditation. The divine cow was Goddess Vishwaroopa Saraswati whom Brahma failed to recognize. Lord Ishaan enlightened Brahma that the Goddess had originated from him. Praying to this form of Shiva helps fulfil all desires and gives enlightenment.
Brahma created beings that were solely of male principle. There was no expansion of creation. He prayed to Shiva who appeared to him as Ardhanarishwara – half male and half female. Left side was the divine mother and the right side was Shiva. Shiva separated the left side or the female principle and instructed Brahma to begin creation with both male and female principle.
To destroy Daksha`s yagna and to put an end to him after the immolation of Sati, Shiva took an avatar of Virabhadra, a ferocious form. He killed Daksha and his army.
Sage Shilada was a great devotee of Lord Shiva and he prayed to Shiva for a son. Shiva granted him the boon that he will be born to them from a sacrificial fire and not of physical union. Lord Shiva took birth as Nandi and did a lot of penance. Nandi has eternal abode in Kailash with Goddess Parvati and Shiva.
Bhairav incarnated from the third eye of Shiva to sever the fifth head of Brahma. The head of Brahma failed to get detached from his hands. He was directed by Shiva to go to Kashi for atonement of severing Brahma`s head. When Bhairav reached Kashi, the skull of brahma used as a begging bowl dropped from him and he was free.
To subdue the wrath of Narasimha, half man- half lion incarnation of Vishnu, Lord Shiva took the avatar of Sharaba or a half bird and half beast. When one prays to this very powerful eight legged divine being, it helps to destroy all forms of negativity, however deep and complex in nature.