Nandaka — Bhagavān's Sword of Divine Knowledge
Nandaka (Sanskrit: nandaka = 'that which causes joy, that which delights', from nanda = joy, bliss) is the name of Bhagavān Viṣṇu/Nārāyaṇa's divine sword (khaḍga or asi). While Bhagavān is most commonly depicted in His four-armed (caturhasta) form holding the conch, discus, mace, and lotus — the sword Nandaka appears in certain expanded iconographic forms, and is named as one of Bhagavān's divine arsenals in the Purāṇic literature. The Viṣṇu Purāṇa (Chapter 21, Amśa 1) names the five principal divine weapons of Bhagavān: Pāñcajanya (conch), Sudarśana (discus), Kaumodakī (mace), Śārṅga (bow), and Nandaka (sword) — each explicitly named as a divine, sentient being serving Bhagavān willingly, not a mere object.
The theological significance of Nandaka is elaborated in the Bhāgavata Purāṇa (12th Skandha, Chapter 11), which presents an allegorical interpretation of all of Bhagavān's attributes: Nandaka the sword is identified as vidyā (knowledge) — specifically the divine knowledge that cuts through ajñāna (ignorance). In this interpretation, the sword represents jñāna-śakti — Bhagavān's power of omniscience and His capacity to illumine truth and dispel falsehood. The name Nandaka — 'that which brings joy' — is appropriate: true knowledge of Bhagavān's nature and one's own relationship to Him (tattva-jñāna) is indeed the source of the supreme joy of liberation. The Bhāgavata allegorizes the entire set of Bhagavān's attributes: the conch is sky-element, the discus is mind, the mace is prāṇa, the lotus is beauty of creation, and the sword Nandaka is knowledge.
In Śrī Vaiṣṇava theology and Viśiṣṭādvaita framework, Nandaka's personification as a conscious, devotion-filled divine being (nitya-sūri) is consistent with the broader teaching that all of Vaikuṇṭha — its residents, its landscape, its very implements — are engaged in joyful kaiṅkarya (service) to Bhagavān. The sword as knowledge (jñāna-khaḍga) also resonates with the Bhagavad Gītā's teaching (Chapter 4): 'jñānāsinācchattvā' — 'having cut asunder with the sword of knowledge' — the doubts and false identifications that bind the soul. The Āchāryas in their upanyāsams (expository discourses) often invoke Nandaka as the symbol of the liberating knowledge that frees souls from saṃsāra.