Chapter 10 of 20 · Rudra Samhita - Srishti Khanda · Overall chapter 42 of 464
Parama Shiva Tattva
Shiva commands Vishnu to take avataras to destroy worldly misery whenever Rudra's own tasks exceed what is possible for him, declares that Rudra and Vishnu are ultimately one and the same, and lays out the precise relative lifespans of Brahma, Vishnu and Rudra - all nested inside Sadashiva's own single breath - before vanishing and establishing the lingam-and-pedestal as the eternal form of his own worship.
Parameshvara said
Hari, Vishnu, excellent in vows, hear another command of mine. You will always be revered and worshipped in all the worlds. Whenever unhappiness arises in the world Brahma made, you will exert yourself to destroy it. I will help you in all hard undertakings. I will kill your enemies when they are unassailable and fierce. Hari, take many avataras and spread your excellent deeds in the world, bent on being the supreme saviour. This Rudra will always have a form with qualities, and doubtless he will do in the worlds the tasks you cannot. Rudra will meditate on you and you on Hara. There is no difference between you and Rudra; you are one. By this boon and by your play you become Maha Vishnu. This is true. If a man devoted to Rudra criticizes you, all his merit will quickly turn to ash, and because of hatred of you he will fall into hell. Vishnu, this will happen by my command. In this world give liberation to men, and especially pleasures. Devotees will meditate on you and worship you. Punish, and show favour.
Brahma continued
Saying this, he took Vidhatri and Hari by the hand.
Shiva continued
In times of distress always help. You will rule everyone. You will give pleasures and liberation. Always be the best at achieving all desired aims. By my command be the breath of life for everyone. Hari, when hardship comes, Rudra, who arose from me, will be worshipped. Whoever takes refuge with you surely takes refuge with me. Whoever sees a difference between the two will certainly fall into hell. Now hear the lifespans of the three gods, Brahma, Vishnu and Hara; there should be no doubt. Four thousand mahayugas make one day of Brahma, and his nights are the same length. Thirty days make a month and twelve months a year. Brahma's life is a hundred years. One of Brahma's years is one day of Vishnu. Vishnu too lives a hundred years. One year of Vishnu is one day of Rudra. After living a hundred years, Hara is established in the form of Nara and stays there as long as Sadashiva breathes in through his mouth. When Sadashiva breathes out, he merges into Shakti. Best of gods, for embodied beings, Brahma, Vishnu, Hara, gandharvas, uragas and rakshasas, a day and night is said to consist of 21,600 breaths in and out. Six breaths make one pala, sixty palas one ghati, and sixty ghatis one day and night. There is no measure of Sadashiva's breaths, so he is called undecaying. It is my command that you keep this order and maintain all that is created, with its many qualities, for that span of time.
Brahma continued
Hearing Shambhu, Bhagavan Hari and I bowed before the lord of the universe. Hari controlled himself and said slowly:
Vishnu said
Shankara, ocean of compassion, lord of the universe, I have heard. By your command I will do all this and always meditate on you; it cannot be otherwise. Earlier I gained all my strength from you. Lord, meditation on your supreme form will never leave my mind, not for the least moment. Lord, if any devotee of mine criticizes you, doubtless he will live in hell for ever. Your devotee will be greatly loved by me. One who knows this will find liberation not difficult. You have certainly increased my greatness. If I lack in qualities, please forgive it.
Brahma continued
Shambhu heard his excellent words and said to Vishnu, "I will happily forgive any lack." With this the ocean of compassion showed Hari and me his compassion and touched all our limbs with his hands. Hara, who takes away all miseries, taught us different kinds of dharma, gave us many boons and told us what was beneficial. Then Shambhu, who loves his devotees and is full of compassion, vanished from the spot as we watched. Since then, Shambhu, giver of pleasure and liberation, has been established in the lingam, and the ways of worshipping an ordinary lingam are said to have been established in the world. Maheshvara is himself the lingam and Mahadevi the pedestal of the lingam. It is said that the whole universe dissolves into the lingam. A person who, in the presence of a lingam, constantly reads this account of the lingam for six months takes Shiva's form; no need to doubt it. Great sage, I cannot speak of the fruits of the good deeds of one who does any work near a lingam.
Retold in our own words from Bibek Debroy's English translation of the Shiva Purana (Penguin Classics). Names, numbers and the order of events follow the source.