If we go to any important Shiva temple we will find a niche on the outer wall of the sanctum sanctorum, exactly behind the spot where the deity is installed. In that niche we can find a representation of the Lingodbhavamoorthi, a form emerging out of a Linga.
We can see neither the top half of the head nor the bottom half of the legs of that form. All the other attributes of Shiva will be found sculptured. We will also find depicted a swan in flight at the top of the Linga and a boar burrowing the earth at the bottom.
According to tradition, Brahma took the form of a swan to find out the crown of Shiva’s head and failed. Similarly, Vishnu took the form of a boar and burrowed deep into the bowels of earth to locate the feet of Shiva and failed.
Thus in Lingodbhavamoorthi we have a unique combination of Brahma, Vishnu and Shiva, impressing in our minds the Advaitic Tatva that God is One, Full and All Pervasive. Both the aspects of Ishavara with form and without form are thus depicted.
The glorious form of the Lingodbhavamoorthi stands out in all majesty to tell us that out of His formlessness Ishvara emerged in a form to guide the functioning of the Universe. When the static Ishvara decides to assume form, he becomes dynamic and manifests Himself in innumerable forms to fulfill His innumerable functions.
The Lord appears in the form of Lingodbhavamoorthi on Shivaratri to shower his grace on us. It is our duty on that day to fast, keep vigil and worship him at midnight at least with one leaf of the Bilva tree. There is a saying that no intelligent dog will touch his food on Shivaratri day.
Sankaracharya of Kanchi