Vedanta declares that our real nature is divine: pure, perfect, eternally free. Our true Self , the Atman, is one with Brahman (the universal consciousness). Maya is the veil that covers our real nature. It also covers the real nature of world around us.
In Vedic literature, Varuna is the cosmic sovereign who upholds the material world. Maya or cosmic illusion/ magic is his primary creative power. He is thus called ‘Mayin’ – the lord who presides over Maya.

In Sankhya & Yoga schools of philosophy, Purusha corresponds to pure unchanging consciousness (soul) and Prakriti is the creative force responsible for the material universe (nature). This also echoes in the later concept of Shiva-Shakti, so familiar to us.
Vedas suggest that the Supreme Being creates the world out of the desire to become many. The cosmos is thus the ‘becoming’ of the Supreme. Indra represents the Supreme Being. Varuna is the non being (material universe).
This corresponds roughly to purusha & prakriti. Varuna thus conceals the true nature of the Supreme through his power of maya. Indra represents the positive aspect of creation and Varuna its opposite and they must work together to keep the universe in harmony.
The Vedas refer to this in the conjoint Indra-Varuna verses. Varuna is called an Asura in the Vedas, which just means powerful. Varuna thus, can be crudely characterized as a negative principle. In the latter Puranas, asuras came to have a negative meaning.

’Rta’ means the natural rhythm that governs the universe. It is derived from the root which means ‘to move.’
Varuna is the sustainer of the material universe. He upholds the law or rta, that keeps the universe going. ‘Dharma’ comes from the root ‘to hold or support’, and roughly means the law that sustains life or that sustains Rta. There is no equivalent word for Dharma or Rta in the English language. The idea of rta is reflected today in the word ‘ritu’ or season.

This quote is paraphrased from the 16th hymn of the 4th book of the Atharva Veda.
The Varuna suktas of the Rig Vedas have strict injunctions against acts of killing, deceiving etc. Because of his love for justice, it is Varuna and not Indra who is exalted as king or ‘samraj’ in the Rig Veda and served as the Vedic model of kingship. He is also merciful.
The Brahmanas describe the Varunapraghasa yajna where barley was offered to absolve one of sins and escape Varuna’s noose. The use of barley, which is one of the oldest cultivated cereals, testifies to the antiquity of the Varuna tradition.
The Yajur Veda calls him the governor of the moral world who carries the pasu (noose) to deter people from wrong actions. In later stories, it Is Yama or the God of Death who judges people and carries the noose.
The Wikimedia image shows a 6th-8th cent CE statue of Varuna with a noose and with a swan.

The Taittirya Upanishad contains the idea of panchakosha as a way to describe human existence. It says that a human can be defined by 5 psychological layers : annamaya kosha (physical body), pranamaya kosha (vital energy), manomaya kosha (mind - emotions & thoughts), vijnanmaya kosha (intellect - decision making) & anandmayi kosha (atman or bliss).
Also, in the Upanishads, Indra is related to jivatman & Varuna to jivaprakriti. Both are identical with the Brahman or Supreme Consciousness. The Vedic deities get subsumed in the Brahman.
The Upanashadic focus shifts to how the creation, sustenance & regeneration of the world is related to Brahman and how an individual can realise the truth of atman’s identity with Brahman.
Thus, we see Vedic Gods being substituted by Brahma-Vishnu-Mahesh and the emergence of yoga, karma & bhakti. The principles behind the Vedic Gods are continued but under a different framework. The image is the cover of a book on Panchakosha model.

In Vedas and Brahmanas, Varuna is intimately associated with the cosmic waters. He is related with the Vedic goddess Apah (water), along with the primordial matter Viraj (shining) and is called Sallilesvara (lord of water). Varuna was the cosmic water in which the sun rested after setting. Manusmriti states that the cosmic water was the first creation of the Supreme Being (Nara) who chose to recline in it (Ayana) – Narayana is thus the Supreme Being who rests in the cosmic waters.
In another etymology, Narayana also means the refuge (ayana) of humans (nara).
There are many great temples in India which depict Vishnu in reclining form. Varuna’s role in the Vedas as lord of cosmic order got shifted to Vishnu in later texts. Vishnu is the sustainer of the cosmos, who descends to earth as avatars to restore Dharma and who serves as role model of leadership through the Rama & Krishna avatars.
The Wikimedia image SSrirammt is of the famous temple at Srirangam, TN where reclining Vishnu is worshipped as Sri Ranganathaswamy.

Gita 10.29 says, “ananatascasmi naganam varuno yadasmaham pitrnamaryama casmi yamah samyamatamaham.”
In this verse, Krishna reveals how His divine presence pervades the best beings in all spheres of life. Varuna is spoken of as the sovereign of water bodies. By the time of the Gita, Varuna had been divested of all his Vedic attributes which had been transferred to other divinities and philosophies.
In the Puranas, he is the lord of earthly water bodies and is depicted as riding a crocodile as his vahan. In the Ramayana, Rama threatens to burn down the sea as it was preventing the vanara army’s crossing to Lanka. A terrified sea god suggests that Ram use the services of the divinely trained engineer Nala, to build a bridge. The sea god is called Sagara and Salila and can be said to refer to Varuna. The image from Britannica.com shows Varuna riding a crocodile.

In addition to his Vedic attribute as the upholder of order, the Yajurveda depicts Varuna as the lord of physicians who is a compassionate healer and uses herbs. In Ayurveda, he is the lord of water and the herb named after him, varuna (also called trinakantaka & marut, scientific name – crataeva nurvala) is a natural treatment for urinary disorders, kidney stones, prostrate problems and excretion of bodily fluids. The image from planetayurveda.com shows the Varuna herb.

In the Mahabharata, Varuna is depicted as the lord of water, son of Adita and the guardian (dikpala) of the western direction. Varuna lives in a magnificent palace underwater along with his wife Varuni.
When the Pandavas are allotted the forested Khandavaprastha, they begin burning it to build their capital, Indraprastha. Indra decides to stop the burning as his friend Takshaka (the snake king) resides in the forest. Indra showers rains to quell the fire. Arjuna prays to Agni (fire god) for a weapon that can help defeat Indra. Agni asks Varuna to give Arjuna the celestial bow Gandiva and an inexhaustible quiver of arrows.
The Itihasa Puranas depict both Indra and Varuna as mighty warriors but who are regularly defeated by others. Indraprastha is said to be the site of the oldest settlement in Delhi. The modern India’s capital has a Varuna connection!
The Wikimedia image by AKS.9955 shows a 8th cent CE basalt statue from Karnataka depicting Varuna with a noose with his wife Varuni.

The Sindhi community celebrates new year as Chet-i-Chand on the first day of the lunar month Chet (Chaitra). It commemorates the birth of Uderolal, more popularly known as Jhulelal, who is regarded as the Patron Saint of the Sindhis.
On this day, people worship water, the giver of life. It is said that Jhulelal worshipped Varuna. The Kolis (fishermen) of Konkan celebrate Narali (coconut) Purnima in the lunar month of Shravan to mark the end of monsoon and the reopening of the sea to fishing. The community reveres Varuna on this day. These two festivals are the living links to Varuna, the great deity of the Vedas. The Wikimedia image by coolross depicts Jhulelal.

Scholars have been struck by the similarity in practices and language of the Vedic & early Iranian people who followed Zoroastrianism. The devas of the Vedas are portrayed as negative and the asuras become positive. The great deity of Zoroastrians is Ahura Mazda (wise lord) – the ‘s’ in Vedic Sanskrit becoming ‘h’ in early Iranian. Like Varuna, he is conceived as the guardian of truth, free from guile and who binds criminals in fetters.
Some say that the first teacher, Zarathustra was repelled by the violence & power of Indra and other devas and elevated the ethical qualities associated with Varuna. The three-fold path of Zoroastrianism exalts Humata (good thoughts), Hukhta (good words) & Hvarshta (good deeds).
In India, the Zoroastrians who initially migrated to India after the Muslim conquest of Iran in 6th-7th cent CE are called Parsis.
The Wikimedia image by A Davey shows a depiction of Ahura Mazda in Taft, Iran.

In the Rig Veda, there are Indra-Varuna as well as Mithra-Varuna hymns. Like Indra & Varuna, Mithra is also part of the Adityas (Sun). Mithra is said to represent the rising sun. Varuna represents the setting Sun who rests in cosmic waters. Mithra regulates human affairs and is sometimes called the god of contracts.

Mithra was an important deity in Zoroastrianism and was widely worshipped in the pre Christian Roman empire in the 2nd-3rd cent CE. The early Romans worshipped the sun in many forms. The official Sun god was Sol Invictus whose birthday on Dec 25th , which was later adopted as Christmas.

Mithraism was a secret cult among soldiers mainly and not much is known about it. Bull sacrifice seems to be a key ritual in Mithraism. The image from worldhistory.org shows the Roman Mithra sacrificing a bull.
https://www.worldhistory.org/Mithra/