Vāsudeva’s Upadeśa: The Inner Enemy and the Indra–Vṛtra Precedent (आत्मशत्रु-बोधः; इन्द्र-वृत्रोपाख्यानम्)
ततो वृत्रं शरीरस्थं जघान भरतर्षभ । शतक्रतुरदृश्येन वज्ञेणेतीह न: श्रुतम्,भरतश्रेष्ठ! तत्पश्चात् शतक्रतुने अपने शरीरके भीतर स्थित हुए वृत्रासुरको अदृश्य वज्के द्वारा मार डाला ऐसा हमने सुना है
tato vṛtraṃ śarīrasthaṃ jaghāna bharatarṣabha | śatakratur adṛśyena vajreṇetīha naḥ śrutam ||
Then, O bull among the Bharatas, Śatakratu (Indra) struck down Vṛtra who had taken up residence within his own body—so we have heard here—by means of an unseen thunderbolt.
वायुदेव उवाच
The verse uses the Indra–Vṛtra episode as an exemplum: when an obstructive force hides within oneself (symbolically, inner corruption or impediment to duty), it must be confronted and removed with resolute, dharma-aligned means, even if the remedy is subtle or not outwardly visible.
Vāyudeva reports a received tradition: Indra (Śatakratu) killed Vṛtra who was lodged within Indra’s own body, using an ‘unseen’ vajra (thunderbolt). The statement is framed as hearsay/authoritative tradition (“so we have heard”).