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There are many stories regarding the birth of universe and about the emergence of the three supreme gods i.e., Trimurti so, it's kind of impossible to know which will be the correct one but I believe every iteration of the story is correct in its own way but I will follow my version which I think is true as its taken from the Bhagavad Gita, Chapter 11, Verse 10-11, when Lord Krishna shows Prince Arjuna his infinite, universal form which also includes Lord Brahma's and Lord Shiva's faces among other faces of his forms. So, we can say the three gods are just energy which have taken different forms due to different needs.
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At the beginning there was nothing in the void except the undifferentiated mass or cosmic ocean, referred to as aprakrita or pralaya (chaos). From this primal state, in the cosmic ocean, God Vishnu, the Preserver, manifested himself as Narayana (who existed before time and space, symbolizing the formless, eternal reality) lying in the primordial ocean of Ksheer Sagar. God Vishnu then created God Brahma, the creator God, from a lotus that sprang from his navel in his cosmic sleep. Brahma, in turn, is responsible for the actual creation of the universe, organizing the primordial elements and laws of nature, birthing beings into a cosmos. Then from the formless void arose God Shiva (the Destroyer) who created himself and so is known as Swayambhu. God Shiva is a timeless deity as he is not born in the conventional sense but emerged from the divine source much like God Vishnu.
God Brahma made beings like Devas (Gods) and Asuras (Demons). Asuras and Devas were born with equal potential however, their paths diverged due to their inherent qualities as the Devas were virtuous, righteous(lawful/Dharma), embodiment of truth and order, representing the forces of creation, preservation, and harmony in the universe. The Devas are led by Indra, the king of the Gods, who has dominium over thunder, rain, and war, and is tasked with the protection of cosmic order. While the Asuras, in contrast were prideful and rebellious. Later they became arrogant and ambitious, the very embodiment of chaos, pride, and unrighteousness (Adharma). So, Devas were tasked with upholding cosmic balance and given the Swarga Lok to rule and reside, while the Asuras due to their ambitions of dominion over the cosmos, and often seeking to usurp the Devas and overthrow their rule confined in the Paataal Lok which did not sit right with the Asura Guru Shukracharya as he believed that Asuras and Devas were the creation of same God and so they should have equal right on the Swarga Lok and they have been neglected by the Supreme Gods due to the Deva's being favored by the Supreme Gods. So Asuras waged war on Deva's for the throne of Swarg Lok to become the King of Gods and for Nectar of Immortality, known as Amrita, to become immortal like the Devas. And so, they waged war against each other resulting in the cosmic level destruction when the Supreme Gods decided to intervene and stop this war among the Asuras and Devas as both were their own creations. But the conflict was not resolved and every yuga cycle they waged war just like now when the Asura Guru Shukracharya is locked in a duel with Indra. Before any God or Asura losses his life in this war the Trimurti decide to intervene. Just when Shukracharya gotten a chance to injure Indra there comes a light so blinding that everybody has to squint their eyes. When the brightness lessens Indra deva and Shukracharya get to see the Trimurti's sacred presence and they all put their weapons down, whether they were Asura or Devas.
All the Deva Clan Members along with Devaraj Indra bowed to the Trimurti and said "Namostu te Vedagarbha, Sarvavidya Pradayaka" (Salutations to the one who holds the Vedas and grants all knowledge), "Om Namo Narayanaya" (Salutations to Lord Narayana), "Om Namah Shivaya" (Salutations to Shiva, the auspicious one).
While the Asuras in the leadership of their leader and Guru, Guru Shukracharya performed Shastanga Namaskara as they did worship God Brahma but for the boons he will give and the Asuras had a particular dislike for God Vishnu as he favors the cosmic balance and is on the righteous side i.e., Devas side but they still showed the much needed respect for one of the supreme beings and Asuras were devoted believer of Mahadev as he is the only one impartial among the three supreme gods.
God Vishnu said "Devaraj Indra why must you continue this senseless war with the Asuras? You are the King of Gods, King of Swarg Lok then what more do you need from the Asuras when they are confined to their Paataal Lok."
Devaraj Indra said with respect "Lord Vishnu it was them who were creating chaos among the sages intervening in the sage's meditation and destroying livestock, so I had to confront them."
Hearing Devaraj Indra slandering the Asuras in front of the Three Supreme Gods while not mentioning the mistreatment of Asuras done by Devas, the Asura Guru Shukracharya too said " Mahadev you have to uphold justice for the Asuras, we were banished to the Paataal Lok from the beginning while the Devas got to rule the Swarg Lok. We were denied the Amrita when we too were involved in the as much as the Devas so Why can't we Asuras get the same treatment as they do? And this war was started by the Asuras and Devas jointly but only we are blamed here."
Hearing the Asura Guru Shukracharya, the Three Supreme Gods were too taken back as they too were not able to judge them with full impartiality so, Mahadev decided on a solution and solemnly announced among the Asuras and Devas "Very soon I will bring into being a new force which will be born from the part of us three Supreme Gods far away from here so as to nobody neither Deva nor Asura can interfere with his destiny and this includes Narayana and Brahmdev. This being that I will bring will be the bearer of justice for all of the creation. He will be the embodiment of impartial justice whether to the Asuras or Devas for he will judge the being on the basis of his karma so I will warn all of you now that you should be careful of what karma you sow as you will soon reap what you have sown."
And after saying this the Trimurti's vanished from the Asura-Deva Battlefield. As Mahadeva have already given the directive so Asuras and Devas too started their own journey to their respective Lok as they have no choice but to stop this war but both Shukracharya and Indra know that while the war may have stopped but the other party will still scheme to get the upper hand for their respective clans.
Earth, India, Mumbai
In a room in a Singhania mansion, there are hurried footsteps of a young man of 25 years old who is pacing back and forth while muttering some incomprehensible words when suddenly a spectacled man places his hand on his shoulder to shook him out of his reverie saying " Aditya, why are you prancing all around here? Don't tell me you are nervous but don't worry everything will be fine. And bro you are the CEO of Singhania Ore Import/Export so it's not befitting for you to be you nervous, and have you decided what to name the child?" Hearing this the man also replies excitedly "What do you know Raghunath except your experiments you science man? Anybody will be nervous when it comes to childbirth so am I. And yes, I and Saanvi have decided on a name for the child if it's a boy I will name him Veer and if it's a girl then I will name her Anuja." Just when they both were talking, there comes a cry of a child, and both stopped talking and don't wait for the room to open they just barges into the room.
Uwaaa.... uwaaa....
On the bed lays a woman so beautiful like a fairy and she doesn't even seem to like she has just given birth to a child. The mid-wife examines the child to determine the gender and inspect if he has any problems or not, seeing both men, the husband and now a father with his friend coming in by forcefully kicking the door open the midwife and the lady on the bed stares at them both. The man then asks the lady about her health "Saanvi dear, are you okay?"
Saanvi, who turns out to the be mother who gave birth to the child replies "Aditya, I am alright."
The man then asks the mid-wife "Tell me is it a boy or a girl?"
The mid- wife replies "Sir Aditya, it's a boy and he is very healthy. Have you decided a name for him."
Hearing the mid-wife say it's a boy all in the room then ignore the mid-wife and Aditya and Raghunath cradles the boy in their arms and after some time they give the boy to Saanvi to see if he is hungry or not. Saanvi takes the child in her arm very carefully and looks at him with care and love and feeds him the by lowering her medical gown from the upper part. Raghunath decides to settle the pay procedure of the doctor and mid-wife while also ensuring the medical procedures of the child and mother are done properly so he leaves the room with the medical staff leaving his friend with his wife and the boy to give them their family time.
Now Aditya and Saanvi both look at the boy who is drinking the milk and Aditya says to Saanvi "Thank You dear you have given me the happiest and greatest gift in the entire world. Now Singhania's have an heir, we will name him Veer, Veer Singhania."
Hearing her husband saying the name of her newborn baby boy Saanvi replies "Yes, Aditya it's our kid and he will someday do wonderful things." Then she whispers in the baby's ears saying " Veer do you like your name? Dad and Mom have chosen this name for you."
And the baby giggles which the duo interpret as the boy's affirmation for the name.