Agni

Agni, also called Agni Deva, is the Hindu god of fire. As the guardian deity of the southeast direction, he is typically found in southeast corners of Hindu temples. In the classical cosmology of Hinduism, fire is one of the five inert impermanent elements along with sky, water, air and earth, the five combining to form the empirically perceived material existence.
In the Vedas, Agni is a major and most invoked god along with Indra and Soma. Agni is considered the mouth of the gods and goddesses and the medium that conveys offerings to them in a homa. He is conceptualized in ancient Hindu texts to exist at three levels, on earth as fire, in the atmosphere as lightning, and in the sky as the sun. This triple presence accords him as the messenger between the deities and humans in the Vedic scriptures. The relative importance of Agni declined in the post-Vedic era, as he was internalised and his identity evolved to metaphorically represent all transformative energy and knowledge in the Upanishads and later Hindu literature. Agni remains an integral part of Hindu traditions, such as being the central witness of the rite-of-passage ritual in traditional Hindu weddings called Saptapadi, in the Upanayana ceremony of rite of passage, as well being part of the diyā in festivals such as Deepavali and Arti in Puja.
Agni is a term that appears extensively in Buddhist texts and in the literature related to the Senika heresy debate within the Buddhist traditions. In the ancient Jainism thought, Agni contains soul and fire-bodied beings, additionally appears as Agni-kumaras or "fire children" in its theory of rebirth and a class of reincarnated beings and is discussed in its texts with the equivalent term Tejas.

Etymology, meaning and other names

Sanskrit अग्नि continues one of two core terms for fire reconstructed to Proto-Indo-European, *h₁n̥gʷnis, other reflexes of which include , the reconstructed name of the fire god in the Albanian pagan mythology, which continues to be used in the Albanian language to refer to Thursday, Latin ignis, Lithuanian ugnis, Kurdish agir, Old Slavonian огнь and its descendants: Russian огонь, Serbian oganj, Polish ogień, etc., all meaning "fire".
The ancient Indian grammarians variously derived it:

from root aj, which in Sanskrit means "to drive" in the sense of "nimble, agile";
from agri, the root of which means "first", referring to "that first in the universe to arise" or "fire" according to Shatapatha Brahmana section 6.1.1; the Brahmana claims this is cryptically called as Agni because everyone including the gods and goddesses are known to love short nicknames;
according to the 5th-century BCE Sanskrit text Nirukta-Nighantu in section 7.14, sage Śakapūṇi states the word Agni is derived from three verbs – from 'going', from 'shining or burning', and from 'leading'; the letter "a" is from root "i" which he claims implies 'to go', the letter "g" is from the root "añj" meaning 'to shine' or "dah" meaning 'to burn', and the last letter is by itself the root "nī" which means 'to lead'.

In the early Vedic literature, Agni primarily connotes the fire as a god, one reflecting the primordial powers to consume, transform and convey. Yet the term is also used with the meaning of a Mahabhuta, one of five that the earliest Vedic thinkers believed to constitute material existence, and that later Vedic thinkers such as Kanada and Kapila expanded widely, namely Dyaus, Vayu, Varuna, Bhumi and Agni.
The word Agni is used in many contexts, ranging from fire in the stomach, the cooking fire in a home, the sacrificial fire in an altar, the fire of cremation, the fire of rebirth, the fire in the energetic saps concealed within plants, the atmospheric fire in lightning and the celestial fire in the sun. In the Brahmanas layer of the Vedas, such as in section 5.2.3 of Shatapatha Brahmana, Agni represents all the gods and goddesses, all concepts of spiritual energy that permeates everything in the universe. In the Upanishads and post-Vedic literature, Agni additionally became a metaphor for immortal principle in humans, and any energy or knowledge that consumes and dispels a state of darkness, transforms and procreates an enlightened state of existence.
Agni is also famously known as:

Pāvaka – one who is the sanctifier of everything;
Havyavāhana – one who carries the sacrificial butter;
Saptajihvi – one who with seven tongues ; and consumes the sacrificial butter very fast;
Vahni – one who is travelling with wind;
Anala – one who is one of the Vasus;
Hutāśana – one who is the consumer of the sacrificed offerings;
Chitrabhānu – one who is the colourful light producing one;
Jvalana – one who is always glowing;
Vaiśvānara – one who is the man of the world;
Vibhāvasu – one whose light is all's wealth;
Dhumaketu – one who is crowned with fire.

Other names include Śikhī, Pingesa, Plavanga, Bhūritejaḥ, Rudragarva, Hiraṇyakṛta.

Origins

There are many theories about the origins of the god Agni, some tracing it to Indo-European mythology, others tracing to Hindu mythology.
The origin myth found in many Indo-European cultures is one of a falcon that carries or brings fire from the deities to people. This messenger also brings an elixir of immortality from heaven to earth. In either case, the falcon returns every day with sacrificial offerings for the deities, but sometimes the falcon hides and disappears to heaven. Agni is molded in similar mythical themes, in some hymns with the phrase the "heavenly falcon that flies".
The earliest layers of the Vedic texts of Hinduism, such as section 6.1 of Kaṭhaka Saṃhitā and section 1.8.1 of Maitrāyaṇī Saṃhitā state that the universe began with nothing, neither night nor day existed, what existed was just the god Prajāpati. Agni originated from the forehead of Prajāpati, assert these texts. With the creation of Agni came light, and with that were created day and night. Agni, state these Saṃhitās, is the same as the Brahman, the truth, the eyes of the manifested universe. These mythologies develop into more complex stories about Agni's origins in the later layers of Vedic texts, such as in section 2.1.2 of the Taittiriya Brahmana and sections 2.2.3–4 of Shatapatha Brahmana.
Agni is originally conceptualized as the ultimate source of the "creator-preserver-destroyer" triad, then one of the trinities, as the one who ruled the earth. His twin brother Indra ruled the atmosphere as the god of storm, rain and war, while his other brother Sūrya ruled the sky and heavens were the other two gods in that trinity. His position and importance evolves over time, in the "creator-preserver-destroyer" aspects of existence in Hindu thought.
The Shatapatha Brahmana mentions there have been three previous Agnis who died and current Agni is the fourth one now.

Texts

Vedas

In the Hindu pantheon, Agni occupies, after Indra, the most important position. Agni is prominent in the hymns of the Vedas and particularly the Brahmanas. In the Rigveda there are over 200 hymns that praise Agni. His name or synonyms appear in nearly a third of 1,028 hymns in the Rigveda. The Rigveda opens with a hymn inviting Agni, who is then addressed later in the hymn as the guardian of Ṛta.
The Vedas describe the foster-parents of Agni as two kindling fire sticks of Prajapati, whose loving action creates him. Just born, he is poetically presented as a tender baby, who needs loving attention lest he vanishes. With care, he sparks and smokes, then flames and grows stronger than his foster-parents, finally so strong that he burns to ashes what created him his residence by Prajapati.
The hymns in these ancient texts refer to Agni with numerous epithets and synonyms, such as Jātaveda, Vaiśvānara, Tanūnapāta, Narāśaṃsa, Tripatsya, and many others. In Hindu mythology, Agni is also presented as one who is mysterious with a tendency to play hide and seek, not just with humans but with the deities. He hides in strange places such as waters, where in one myth, he imbues life force into fishes that dwell therein, due to which the fishes report his presence to the deities, who take Agni to heaven.
Agni is in hymn 10.124 of the Rigveda, a Rishi and along with Indra and Sūrya makes up the Hindu trinity of gods who create, preserve, destroy.
Agni is considered equivalent to all the deities in the Hinduism, which formed the foundation for the various non-dualistic and monistic theologies of Hinduism. These theme of equivalence is repeatedly presented in the Vedas, such as with the following words in the Maṇḍala 1 of the Rigveda:

They call it Indra, Mitra, Varuna, Agni,
and he is heavenly-winged Garuda.
To what is One, sages give many a title,
they call it Agni, Yama, Matarisvan.

— Rigveda 1.164.46, Translator: Klaus Klostermaier

Upanishads

Agni features prominently in the major and minor Upanishads of Hinduism. Among the earliest mention is the legend of a boy sage named Satyakāma Jābāla, the son of an unwed father and an unwed mother, in chapter 4 of the Chhāndogya Upanishad. He honestly admits his poverty and that his mother does not know who his father was, an honesty that earns him a spot in a Hindu school. During his studies, the boy sage meets Agni, who then becomes the god for him as a cardinal direction, world body, eye and knowledge, and the abstract principle of Brahman which the Upanishad states is in everything and is everywhere and he becomes a boy sage. Agni appears in section 1.13 of Chandogya Upanishad as well.
In verse 18 of the Isha Upanishad, Agni is invoked with, "O Agni, you know all the paths, lead me on to success by the good path, keep me away from the wrong path of sin". In sections 4.5–6 of the Maitri Upanishad, the students ask their Hindu Guru Maitri about which deity is best among deities they name, a list that includes Agni. The Guru replies that they are all supreme, all merely forms of the Brahman, the whole world is Brahman. So pick anyone, says the Upanishad, meditate and adore that one, then meditate over them all, then deny and discard the individuality of every one of these deities including of Agni, thus journey unto the universal reality, for a communion with the Purusha, which is the Ātman.
Sections 3 and 4 of Kena Upanishad, another major ancient Upanishad, presents a story which includes gods Agni, Vayu, Indra and goddess Parvati. After a battle between good deities and evil demons, where the deities kill all the demons and win, the deities wonder, "what is this Brahman, a wonderful being?" Agni goes first to find out, but fails. Vayu too goes to find out and fails. Then Indra tries and fails, but meets the Parvati who already understands Brahman, explains what Brahman is and how the deities reached victory through the nature of Brahman. Indra shares this knowledge with Agni and Vayu. The Kena Upanishad closes these sections by stating that "Agni, Vayu, Indra" are revered first because they were the first among the deities to realize Brahman from Parvati. The allegorical legend, states Paul Deussen, aims to teach that all the Hindu deities and natural things have their basis in the timeless, universal monistic principle called Brahman.
Another ancient major Hindu scripture named Prashna Upanishad mentions Agni in its second Prashna. The section states that Agni and other deities manifest as five gross constituents that combine to make the entire universe, and that all the deities are internalized in the temple of a living body with Agni as the eyes.
Agni is mentioned in many minor Upanishads, such as the Pranagnihotra Upanishad, the Yogatattva Upanishad, the Yogashikha Upanishad, the Trishikhibrahmana Upanishad and others. The syncretic and monistic Shaivism and Shaktism text, namely Rudrahridaya Upanishad states that Shiva is same as Agni, and Parvati is same as Svaha.
Mundaka Upanishad mentioned the seven tongues of Agni as kālī, karālī, manojavā, sulohita, sudhāmravarṇā, sphuliṅginī, ''visvarucī''.