Mantreshwara is the god of mantra in Kashmir Shaivism. Tantras are monotheistic; gods are aspects of the Supreme Deity and are generated by His Shakti. Each deity has two forms, the visualized and the sonic, that is, mantra. The sonic or mantra form is closer to the Ultimate Reality and is more potent.
The Supreme God himself 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 utterer. Each and every matrika (a, aa, etc) represents divine energy.
The entire range of the knowable is divided into two classes, the acceptable and the avoidable. The acceptable are Shiva, Shakti, Vidyesahas, mantra, Mantreswara and the jivas. The avoidable are impurity, nescience, actions, maya and the world produced by maya (Malinivijayottara Tantra, 1.15f).
Success in the sphere of the 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 whom He assigns the functions of sustenance, etc, of the universe. Some of these beings are Shiva himself, mantra mahesha, mantresha and mantreswara and mantra.
One hundred and eighteen Rudras are appointed by Mantreswara, and they are headed by Angusthamastra. 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 all creatures, ability to achieve anything, and poetic faculty. Rudra Shakti has three inspirations, Anava, Shakta and Sambhava. Shakti has three aspects – apara or ghoraratri, parapara or ghora and para or aghora. When the Lord feels the impulse for manifesting the world, his inseparable shakti evolves as jnana shakti (cognition) and kriya shakti (activity). The Supreme Reality is considered to be union of Shiva and Shakti (yamala).