स वै वृत्र इति प्रोक्त: पाप: परमदारुण: ॥ १८ ॥
sa vai vṛtra iti proktaḥ
That very fearful demon, who was actually the son of Tvaṣṭā, covered all the planetary systems by dint of austerity. Therefore he was named Vṛtra, or one who covers everything.
In the Vedas it is said, sa imāḻ lokān āvṛṇot tad vṛtrasya vṛtratvam: because the demon covered all the planetary systems, his name was Vṛtrāsura.