As we know that according to Advaita, Shiva and Vishnu are both Brahman. Is Shakti also a Brahman or is Maya (illusion) of Brahman?
The Advaita Vedanta formulation of Sankaracharya considers Maya to be anirvachaniya or inexplicable and unknowable.
It is ignorance of Brahman that gives us the experience of the world.
Advaita Vedanta says that ignorance of Brahman has created this world. Ignorance, which is the opposite of knowledge (jnana-virodhi), is not something negative. It has a positive character (bhavarupa). It has two powers: (1) the veiling power (avarana-shakti) and (2) the power of projection (vikshepa-shakti). Ignorance has the power to cover or veil the Reality, which is Brahman, and also the power to project this world on the veiled or covered Reality. The world is the product of mula-avidya. The individual ego, which is part of this world, is also the product of mula-avidya. Consciousness associated with this ego is called the Atman or the Jivatman. Between the Jivatman and Brahman (Paramatman) there is a thin veil of ignorance. It is like a cloud that does not all an individual to see the sun. In this analogy the Brahman is the sun and the patch of cloud is the ignorance of the individual. When this individual’s ignorance is dispelled by the knowledge of Brahman, he or she will have the experience of Brahman.
Maya and Creation
From the point of view of Advaita Vedanta, maya and avidya are the same. Just as avidya or ignorance has the powers, as it were, to hide Brahman and project something else, such as the world, on it, so also maya, as it were, can cover up Brahman and project the world onto it. Maya cannot really cover the Reality that is Brahman. A patch of cloud can never cover the sun. It only covers the eyes of those who are looking at the sun. In the same way maya covers our knowing ability and thus obstructs our knowledge of Brahman. In any act of knowing, the ‘knower’ and the object known’ have to be separate from each other. They cannot be the same. In order for us to know maya, it has to be separate from us. Individuals like us are the products of maya and we are all posited within the domain of maya. Maya and we individuals are not separate from each other, just as chocolate and a bear-shaped chocolate candy are not different from each other. As we are identical with maya, we cannot know maya. For this reason, maya is not only unknowable; it is inexplicable as well (anirvachaniya).
Journey from Many to One essentials of Advaita Vedanta by Swami Bhaskarananda
Sankaracharya's formulation does not answer the question raised here.
Sri Ramakrishna considers Sakti to be Brahman.
VIJAY: "If Brahman is our Mother, then has It any form or is It formless?"
MASTER: "That which is Brahman is also Kali, the Mother, the Primal Energy. When inactive It is called Brahman. Again, when creating, preserving, and destroying, It is called Sakti. Still water is an illustration of Brahman. The same water, moving in waves, may be compared to Sakti, Kali. What is the meaning of Kali? She who communes with Maha-Kala, the Absolute, is Kali. She is formless and, again. She has forms. If you believe in the form-less aspect, then meditate on Kali as that. If you meditate on any aspect of Her with firm conviction, She will let you know Her true nature. Then you will realize that not merely does God exist, but He will come near you and talk to you as I am talking to you. Have faith and you will achieve everything. Remember this, too. If you believe that God is formless, then stick to that belief with firm conviction. But don't be dogmatic: never say emphatically about God that He can be only this and not that. You may say: 'I believe that God is formless. But He can be many things more. He alone knows what else He can be. I do not know; I do not understand.' How can man with his one ounce of intelligence know the real nature of God? Can you put four seers of milk in a one-seer jar? If God, through His grace, ever reveals Himself to His devotee and makes him understand, then he will know; but not otherwise.
"That which is Brahman is Sakti, and That, again, is the Mother. He it is, says Ramprasad, that I approach as Mother; But must I give away the secret, here in the market-place? From the hints I have given, O mind, guess what that Being is!
Ramprasad implies that he has known the truth of Brahman. He addresses Brahman as Mother.
"In another song Ramprasad expresses the same idea thus:
Knowing the secret that Kali is one with the highest Brahman, I have discarded, once for all, both dharma and adharma.
Adharma means unrighteous actions, actions forbidden by religion. Dharma means the pious actions prescribed by religion, as, for instance, charity to the poor, feeding the brahmins, and so on."
VIJAY : "What remains if one renounces both dharma and adharma?"
MASTER: "Pure love of God. I prayed to the Divine Mother: 'O Mother, here, take Thy dharma; here, take Thy adharma; and give me pure love for Thee. Here, take Thy virtue; here, take Thy vice; and give me pure love for Thee. Here, take Thy knowledge; here, take Thy ignorance; and give me pure love for Thee.' You see, I didn't ask even for knowledge or public recognition. When one renounces both dharma and adharma, there remains only pure love of God — love that is stainless, motiveless, and that one feels only for the sake of love."
A BRAHMO DEVOTEE: "Is God different from His Sakti?"
MASTER: "After attaining Perfect Knowledge one realizes that they are not different. They are the same, like the gem and its brilliance. Thinking of the gem, one cannot but think of its brilliance. Again, they are like milk and its whiteness. Thinking of the one, you must also think of the other. But you cannot realize this non-duality before the attainment of Perfect Knowledge. Attaining Perfect Knowledge, one goes into samadhi, beyond the twenty-four cosmic principles. Therefore the principle of 'I' does not exist in that stage. A man cannot describe in words what he feels in samadhi. Coming down, he can give just a hint about it. I come down a hundred cubits, as it were, when I say 'Om' after samadhi. Brahman is beyond the injunctions of the Vedas and cannot be described. There neither 'I' nor 'you' exists.
"As long as a man is conscious of 'I' and 'you', and as long as he feels that it is he who prays or meditates, so long will he feel that God is listening to his prayer and that God is a Person. Then he must say: 'O God, Thou art the Master and I am Thy servant. Thou art the whole and I am a part of Thee. Thou art the Mother and I am Thy child.' At that time there exists a feeling of difference: 'I am one and Thou art another.' It is God Himself who makes us feel this difference; and on account of this difference one sees man and woman, light and darkness, and so on. As long as one is aware of this difference, one must accept Sakti, the Personal God. It is God who has put 'I-consciousness' in us. You may reason a thousand times; still this 'I' does not disappear. As long as 'I-consciousness' exists, God reveals Himself to us as a Person.
"Therefore, as long as a man is conscious of 'I' and of differentiation, he cannot speak of the attributeless Brahman and must accept Brahman with attributes. This Brahman with attributes has been declared in the Vedas, the Puranas, and the Tantra, to be Kali, the Primal Energy."
The Gospel of Sri Ramakrishna, Visit to Sinthi Brahmo Samaj