Mantreswara is the god of mantra in Kashmir Shaivism. Tantras are monotheistic; gods are aspects of the Supreme Truth and are generated by His Shakti. Each god has two forms, the visualized and the sonic, i.e., mantra.
The mantra form of a deity is closer to the ultimate reality and is more potent. The Supreme Truth has a sonic form, nada, which is the transcendent matrix of all languages and of all realities.
Mantras if practiced according to the tantric precepts, bring the desired results to the one who chants them. Each and every matrika (a a, etc) represents divine energy.
The entire range of the knowable is divided into two classes in Kashmir Shaivism, the acceptable and the avoidable.
The acceptable are Shiva, Shakti, Vidyesas, mantra, Mantreswara, and the jivas. The avoidable are impurity, nescience, actions, maya and the world produced by maya (Malinivijayottara Tantra 1.15 f).
Success in the spheres of matter and self is provided by the right discernment of the acceptable and the avoidable.
The Lord evolves out of Himself the eight pure beings called Vijnana Kevalas to who He assigns the functions of sustenance, etc, of the universe.
Some of these beings are Shiva himself, mantra mahesha, mantresha ormantreshwara and mantra.
One hundred and eighteen Rudras are appointed Mantresvara, and they are headed by Angusthamatra.
The Guru (spiritual teacher), who knows all principles properly, is equal to Shiva.
The Guru is endowed with Rudra Shakti; he has mantra siddhi, control over the creatures, ability to achieve anything, and poetic faculty.
Rudra Shakti has three inspirations, Anava, Shakta and Sambhava.
Shakti has three aspects, namely, apara or ghoratri, parapara or ghora, and para or aghora. When the Supreme Truth feels the impulse of manifesting in the world, his inseparable Shakti evolves as Jnana Shakti (cognition) and Kriya Shakti (activity). The Supreme Reality is considered to be the union of Shiva and Shakti (Yamala).
Bibliography
Medieval Religious Literature In Sanskrit In A History Of Indian Literature Vol II Fasc 1. (1977) by Jan Gonda Published by Wiesbaden Otto Harrassowitz
Hindu Tantric and Sakta Literature. In A History Of Indian Literature Vol II Fasc 1. (1981) by Sanjukta Gupta and Teun Goudriaan Published by Wiesbaden Otto Harrassowitz
Encyclopedia of Hinduism Volume VII page 31 - IHRF