The Advaita School of spiritual philosophy was founded in Kashmir by Sage Vasugupta in the last quarter of the eight century CE. It is said that he was an ardent devotee of Shiva Bhagavan and was very keen to learn the Shaiva mode of spiritual practice. But he could not find a suitable guru, who could satiate his thirst for learning the Shaiva mode of spiritual practice. His intense yearning for learning the spiritual philosophy of Advaita Shaivism and the mode of their spiritual practice to realise his true nature was fulfilled by Mahadev Shiva himself, who appeared before him in his dream and told him: ‘Here, on this mountain (Mahadeva mountain), there is the secret esoteric teaching under a big stone. Having obtained it, reveal it to those who are fit for receiving [my] Divine Grace.’
On waking up next morning, Sage Vasugupta went to the mountain and by a mere touch that particular stone slab turned upside down, and he found the Shiva Sutra inscribed on it. The mountain and the stone slab still exist in the valley of Harvan stream, behind the Shalimar Gardens, near present- day Srinagar in India. Thus, Vasugupta was not the author of Shiva Sutra. It is a revealed text, technically called Agama.
Other works belonging to the category of Agama and also those that are oft-quoted in the later works describing the spiritual philosophy of this school are the Malini Vijaya Tantra, the Svachchhanda Tantra, the Netra Tantra, the Vijnana Bhairava Tantra, and the Rudrayamala Tantra.
Source - article titled 'Light on Abhinavagupta’s Contribution to the Advaita Shaiva Spiritual Philosophy of Kashmir' by Dr Debabrata Sen Sharma published in Prabuddha Bharata Magazine January 2016 Issue.