• ilvalasya sutaḥ — son of Ilvala
  Ilvala — the brāhmaṇa-killing asura from Mahābhārata Vana-parva. The lineage is explicit: Balvala inherits a family vocation of defiling brāhmaṇas. Asura-dharma runs in his blood.
• ghoraḥ — fearsome
  From √ghur. Standard Bhāgavatam epithet for high-grade asuras. Ghora-rūpa, the terrible form. Balvala fits the category.
• balvalaḥ — Balvala
  Name of the demon. From bala (strength) — but as 'Balvala' it doubles the bala morpheme, suggesting brute redundant force. The Lord's elder brother is Balarāma, the right kind of bala; the demon's name is the wrong one.
• dūṣayati — pollutes
  Causative of √duṣ. The verb of contamination of yajña-ground. The opposite of pavitra-karaṇa. Balvala's whole activity is dūṣaṇa; the Lord's whole activity is pāvana.
• parvaṇi parvaṇi — at every new-moon
  Locative-iterative. Parvan = the joint of the month, especially new and full moons. Balvala's attack is calendrical — a counter-rite synchronized with auspicious days. The Bhāgavatam shows asuras using śāstric time against śāstric ends.

Balvala, son of Ilvala, comes every new-moon day and pours pus, blood, feces, urine, wine, and meat upon the sages' sacrifice. The yajña-ground itself is being contaminated rhythmically — a parva-rite of asuric counter-yajña. The sages ask for relief, and they ask the Lord whose feet are pure to do the cleansing.