The Shaiva Linga Mahapurana mentions the Shaiva Itihasa in Chapter 1.26
For the propitiation of eighteen Puranas beginning with Brahma, for the propitiation of eighteen Upapuranas beginning with Saura, and for the propitiation of holy Itihasas beginning with Saiva, he shall touch his ears and the cardiac region. (Verses 26-28)
This Shiva Rahasya Itihasa is supposed to comprise of approximately 100,000 verses and it was published in entire 30 volumes. So it is about the same size as Mahabharata!!
Can anyone tell the outline of the story, or part of the story, in brief or very short way? Like who are the main characters, what happened etc.