What are the benefits of worshipping Indra ?
What are some the notable stotras and mantras that wen chant about Indra which are not originated from vedas and even non initiated persons can chant it ?
Indra (इन्द्र) is the king of the devas (god-like deities) and Svarga (heaven) in Hinduism. He is associated with the sky, lightning, weather, thunder, storms, rains, river flows, and war.
Sakra (शक्र) is a Sanskrit word meaning ""mighty"" or "powerful," and is used as an epithet of Indra in hymn 5.34 of the Rigveda. Indra is praised as the highest god in 250 hymns of the Rigveda – a Hindu scripture dated to have been composed sometime between 1700 and 1100 BCE. He is co-praised as the supreme in another 50 hymns, thus making him one of the most celebrated Vedic deities.
Indra's iconography shows him wielding a lightning thunderbolt weapon known as Vajrāyuda (वज्रायुध/வஜ்ராயுதம்), riding on a white elephant known as Airavata (ऐरावत/ஐராவதம்). Indra's abode exists in the capital city of Svarga, Amaravati, though he is also associated with Mount Meru (मेरु पर्वत/aka Sumeru/மேரு மலை).
Vedic Indra should be worshiped either through Moola Mantra or with Gāyatri mantra.
I) The Gāyatri mantra is:
ॐ देवराजाय विद्महे वज्रहस्ताय धीमहि। तन्नो शक्रः प्रचोदयात्॥
Om Devaraajaaya vidmahey vajrahastaaya dhimahi। Tanno Shakrah prachodayaat॥
Meaning: Om let us meditate upon the King of Devas. May that great God who holds the thunderbolt in his hand, inspire and illumine our mind and understanding.
— Linga Purana.
Anyone can worship Indra with Gāyatri mantra with Bakthi Bhāvanā(भक्ति भावना) and needs no initiation (मन्त्र दीक्षा/Mantra Dīksha).
II) The Moola Mantra of Indra is an Āgamic mantra (or Tāntric mantra) from Mantradevaprakāśikā (मन्त्रदेवप्रकाशिका) as follows:
ऐं इन्द्राय नमः।
Im Indrāya namaha।
1.Rishi(ऋषि/Seer/मंत्र द्रष्टा/Who discovered Mantra) : Brahmā (ब्रह्मा). 2.Chandas(छन्दस्/Metrical Structure of Mantra): Pangti(पंक्ति). 3.Devatha(देवता): Indra (इन्द्र)
— Mantradevaprakāśikā
Tāntric Sādhana of Indra through Moola Mantra Japa requires Mantra Dīksha (मन्त्र दीक्षा/Mantra Dīksha) from a competent Guru(सिद्ध गुरु/Siddha Guru).
In the realm of Tantra, it is hard to find a competent Guru (the one who has perfected/acquired already the Mantra Siddhi for Indra). So the procedure for adopting Sri Dakshinā Moorthi as Guru has been provided in my Quora post for Sri Rāma Tantra (श्रीराम तन्त्र/ஸ்ரீராம தந்திரம் which can be adopted for Tāntric Upāsana of Indra.
Conclusion:
Indra was widely worshipped in Tamil Nadu, where a big festival called Indra Vizha(இந்திர விழா/Festival of Indra) was celebrated in His honor by Choza Kings in Pūmpukar. Agricultural communities worship him to bring timely rains. Devotion to him brings human prosperity and happiness.
It is also well known in Mahābhāratam, the Yadava clan worshipped Indra for rains and Lord Krishna’s childhood involved the lifting of Govardhan Hill.
Good Luck!