Seshnag is poorva rudra and a destructive aspect of Vishnu . He is the destructive aspect of Sankarshana vyuha of Vishnu
Vishnu Purana mentions that at the end of kalpa he destroys the universe.
- कल्पान्ते मुखतो यस्य विषवह्निर्विनिःसृता ।
- त्रैलोक्यं दहते रौद्रं रूपं कृत्वा सुदारुणम् ॥
- At the end of every aeon, Rudra, who’s one with Saṃkarṣaṇa, … emerges from the mouths of Śeṣa to devour the threefold worlds.”
As per Mahabharata, Sankarshana vyuha is involved in the destruction of creation.
संकर्षणः शेष इति प्रोच्यते...
- “He that enters the body … is called Jīva. He is known also by other names, viz., Sesha and Sankarshana.”
- ततोऽहं पुनरेवात्र सृष्टिं सृजामि च प्रभो ।
- मम चतुर्थं तु रूपं सृजत्येष सनातनम् ।
- शेषाख्यं तु महाबाहो संकर्षणमुदाहृतम् ॥
- tato'haṃ punar evātra sṛṣṭiṃ sṛjāmi ca prabho |
- mama caturthaṃ tu rūpaṃ sṛjaty eṣa sanātanam |
- śeṣākhyaṃ tu mahābāho saṃkarṣaṇam udāhṛtam ||
- “Afterwards I again create creation here. My fourth form creates the eternal one called Śeṣa; that Śeṣa is called Saṃkarṣaṇa.”
One of the oldest description of Sankarshana Balaram in 2th BCE in mathura is shown to have thousand headed serpent as hood.
In Valmiki Ramayana, he bears weight of the entire earth from Patala and sustains the three worlds and all the gods combined