Vrtrasura (वृत्रासुरः)

Vrttrasura (Samskrit : वृत्रासुरः) is the son of Tvashtra Prajapati. त्वष्ट्रा॥ Tvashthra angered by Indra's killing of his son विश्वरूप || Vishwaroopa (also called Trisiras), initiates an आभिचारिकं यागं || aabhaicharikam yagam (a yagam performed to cause harm to an intended person) to procure a son to kill Indra (Devi Bhagavatam (Skanda 6). However, due to the mispronunciation of the swara of mantra "इन्द्रशत्रुर्वर्धस्व", the entire meaning of the mantra was altered (Devi Bhagavatam, Skanda 6) . Instead of "let there be a son who could kill Indra" it meant "let there be a son who will be killed by Indra". Thus, was born Vrttrasura, who eventually was killed by Indra. Rig veda mantras dedicated to Indra's might and valor in killing Vrttra are plenty in number.

Vedas describe a continuous battle between  Asuras (demons) and the Suras (devas).  There is no other personality or subject that is accorded more importance than Vrttra and the Vrttrasura vadha (killing of Vrttra).

व्युत्पत्तिः || Etymology

From origin perspective Vrttra is Tvaashtra, the son of Tvashtra. However, Vrttra has been defined by Yaska maharshi as follows

तत्को वृत्रः। मेघ इति नैरुक्ताः । त्वाष्ट्रोऽसुर इत्यैतिहासिकाः । अपां च ज्योतिषश्च मिश्रीभावकर्मणो वर्षकर्म जायते । तत्रोपमार्थेन युद्धवर्णा भवन्ति । (Nirukta 2.16)

tatkō vr̥traḥ। mēgha iti nairuktāḥ । tvāṣṭrō'sura ityaitihāsikāḥ । apāṁ ca jyōtiṣaśca miśrībhāvakarmaṇō varṣakarma jāyatē । tatrōpamārthēna yuddhavarṇā bhavanti । (Nirukta 2.16)

Meaning : Nirukta defines Vrttra as the cloud, which is a water body. A combination of water and lightning (electricity) gives rise to the process of rain and this process is used as a simili to describe a war.  The Taittiriya samhita describes Vrttra as follows 

....इषुमात्रमिषुमात्रव्विँष्वङ्ङवर्धत स इमाल्लोँकानवृणोद्यदिमाल्लोँकानवृणोत्तद्वृत्रस्य वृत्रत्वन्तस्मादिन्द्रोऽबिभेदपि.... (Arsheya system 4.5.36.2 and Saraswat System 1.2.4.12.2)

Meaning : Vrttra is the one who grows circularly, who goes in circles and occupies the worlds.  

वृत्रस्य पूर्वजन्म || Vrttra's Previous Birth

Chitraketu ruled over Soorasena kingdom. Chitraketu and his wife Kritadyuti were childless. Seeing their sorrow Angirasa maharshi informs them that a son would be born to them. However, the envy of Chitraketu's other wives led them to poison the child and kill him at birth. Angirasa and Narada tried to revive the child but the soul replies that he was the son to a large number of parents, in various births, and requested to be enlightened as to which of those parents he was to live with. Understanding the situation, Narada leaves after imparting spiritual wisdom to Chitraketu and his wife. Both of them then immerse themselves in the spiritual path and attain the form of Gandharvas. Once in the land of Vidyadharas, Chitraketu was travelling in the aerial space in the vehicle given by SriMahavishnu. When flying over the Mountain Kailasa, the abode of Shiva and Parvati, Chitraketu laughs at the sight of Parvati being seated in the lap of Shiva. He accuses Shiva, being enlightened is embracing his consort, Parvati, when present in the view of Siddhas and saintly people as an ordinary human being. This anecdote is given in Bhagavata Purana in 6th Skanda as follows

जटाधीस्तीव्रतपा ब्रह्मवादिसभापतिः अङ्कीकृत्य स्रियं चास्ते गतह्रीः प्राकृते यथा || (Bhag. Pura. Skan. 6. Chap 17. Slok 7)

प्रायशः प्राकृताश्चापि स्रियं रहसि बिभ्रति । अयं महाव्रतधरो बिभ्रर्ति सदसि स्रियम् || (Bhag. Pura. Skan. 6. Chap 17. Slok 8)

Enraged by his laughter, Parvati curses Chitraketu to be born as an asura.

नायमर्हति वैकुण्ठ पादमूलोपसर्पणम् सम्भावितमतिः स्तब्धः साधुभिः पर्युपासितम् (Bhag. Pura. Skan. 6. Chap 17. Slok 15)

अतः पापीयसीं योनिमासुरीं याहि दु्र्मते यथेह भूयो न कर्ता पुत्र किल्बिषम्

जज्ञे त्वष्टुर्दक्षिणाद्नौ दानवीं योनिमाश्रितः वृत्र इत्यभिविख्यातो ज्ञानविज्ञानसंयुतः (Bhag. Pura. Skan. 6. Chap 17. Slok 38)

Thus Chitraketu was born as Vrttrasura.

सप्तसिन्धवः || Saptasindhus (The Seven Rivers)

Rig Veda describes the following about Vrttrasura and his blocking the Sapta sindhu's (the seven rivers).

अतृप्णुवन्तं वियतमबुध्यमबुध्यमानं सुषुपाणमिन्द्र | सप्त प्रति प्रवत आशयानमहिं वज्रेण वि रिणा अपर्वन् || (Rig. Veda. 4.19.3)

atr̥pṇuvantaṁ viyatamabudhyamabudhyamānaṁ suṣupāṇamindra | sapta prati pravata āśayānamahiṁ vajrēṇa vi riṇā aparvan || (Rig. Veda. 4.19.3)

Meaning : Vrttra having an insatiable bulky body, is very powerful, personification of ignorance, and deep in slumber. Serpent like he sleeps in a way that the sapta sindus can’t flow. Such Vrttra is killed by Indra with his thunderbolt, on the day of full moon.

Esoteric Meaning : Here the birth and killing of Vrttra represent an eternal principle that happens in this world. Water is the main life force that keeps things green, without water nothing survives, drought occurs. Similarly in the personality of human beings, if the flow of life or the sapta sindus stop, then there is a drought of spiritual existence & elevation. Every aspect of human beings, the emotions, intellectual capacity, friendship, values or relationships are affected by the lack of spiritual elevation which is a drought-like situation. Anything which stagnates brings about decay and destruction.

In this way the flow of ruthu or season has to continue which is the nature of this world. One who is on the ruju marga (path of righteousness) seeks to go with the flow of spiritual elevation and to lie down and sleep against this flow of water is the important activity of Vrttrasura. Panis steal spiritual knowledge, but Vrttra is the one who stops the flow of life. He is the one who lays down obstacles for those who seek to achieve higher realms of life and that gives him happiness.

This relationship of Vrttrasura as one who seeks to impede water flow is given in the following Rig Veda mantras

समुद्रज्येष्ठाः सलिलस्य मध्यात्पुनाना यन्त्यनिविशमानाः | इन्द्रो या वज्री वृषभो रराद ता आपो देवीरिहमामवन्तु || (Rig. Veda. 7.49.1)

samudrajyēṣṭhāḥ salilasya madhyātpunānā yantyaniviśamānāḥ | indrō yā vajrī vr̥ṣabhō rarāda tā āpō dēvīrihamāmavantu || (Rig. Veda. 7.49.1)

Meaning : The river waters flow continuously, purifying the entire existence on their way to reach the chief of waters, the Ocean. Oh Indra! the one who wields the Vajra (Vajraayudha) free the waters, (as you are the) one who freed the cows. Protect me just as you freed the rivers (by removing the impediment of Vrttra) and freed the cows (by removing the impediment of Vala and Panis).

वृ॒त्रः क्षु॒त्खलु॒ || Vrttra is Hunger

Tvashtra prajapathi first had a son named Vishwarupa. Once the devaguru, Brihaspati, being angry, left his priestly work and so the devatas invited Vishwarupa to conduct their activities as their priest. Vishwarupa, (whose mother belonged to Asuras) was favorable to the asuras and without the devatas knowledge, he offered havishya (oblations) to the asuras which rightfully belonged to devatas. When Indra came to know about this, he was furious and killed Vishwarupa. Hearing this,Tvashtra was very angry and performed a yajna to get a son who could kill Indra.

While chanting the mantras due to wrong pronunciation the purport changed. Instead of "let there be a son who could kill indra", it became "let there be a son who will be killed by Indra". Instantly, Vrttra was born and he grew over the expanse of the universe in circular fashion. Indra with the blessing of Mahavishnu then defeated Vrttra who entered into a truce with Indra.

इ॒दमस्मि॒ तत्ते॒ प्र दास्या॒मीति॒ त्वी (३) इत्य॑ब्रवीत्स॒न्धान्तु सं द॑धावहै॒ त्वामे॒व प्र वि॑शा॒नीति॒ यन्माम्प्र॑वि॒शेः किम्मा॑ भुञ्ज्या॒ इत्य॑ब्रवी॒त्त्वामे॒वेन्धी॑य॒ तव॒ भोगा॑य॒ त्वाम्प्र वि॑शेय॒मित्य॑ब्रवी॒त्तव्वृँ॒त्रः प्रावि॑शदु॒दरं॒ वै वृ॒त्रः ख्षुत्खलु॒ वै म॑नु॒ष्य॑स्य॒ भ्रातृ॑व्यो॒ यः [41] (Tait. Samh. Arsheya system 4.5.36.6 and Saraswat System 1.2.4.12.6)

Summary : Vrttrasura said allow me to enter into your body. Indra asked him what will you do with me? Will you eat me? Vrttra replied that he will become the fire in the stomach, as hunger (jataragni) in human beings.

In this way Vrttra entered into the body of Indra and the living entities and continues to stay in the form of hunger. He is called as भ्रातृ॑व्यो॒ brathravya (those who understand conquer hunger.  

Esoteric Meaning : It should be noted that Vrttra was though killed in historic perspective, was not killed but was given a place in every living entity as hunger. Therefore the disturbance to the living entity is an eternal experience in this world, symbolically, hunger is the sign of dissatisfaction. Similarly, hunger is not just limited to physical hunger but is connected to insatiable wants and desires. Lust and anger are insatiable hungers.  

you could add here the reigning in the waters by Indra & How V controls the waters  

References

1. Narayanacharya, K. S. (2011). Veda Samskrita Parichaya. Hubli:​Sahitya Prakashana​.
2. Rig Veda mantras link : http://ignca.nic.in/Vedic_portal_rigveda_shakala_Samhita_m04.htm
3. Puranic Encyclopedia link : https://ia802606.us.archive.org/17/items/puranicencyclopa00maniuoft/puranicencyclopa00maniuoft.pdf
4. Krishna Yajurveda Samhita (Taittiriya Samhita) link : http://parankusa.org/KrYajurBrowse.aspx and http://sanskritlibrary.org/download/krishna_yajur_ved.html
5. Bhagavata Purana. Gita Press