ईदग्नेः सोमवरुणयोः

Operational rule vidhi

For the final of अग्नि is substituted the long ई when 1. सोम or 2. वरुण follow.

Word by word (padaccheda)

Substitute / term ईत् prathamā · ekavacanaTarget अग्नेः ṣaṣṭhī · ekavacanaRight context सोम- वरुणयोः saptamī · dvivacana

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").

ईत् अग्नेः सोम-वरुणयोः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

अग्नेः → ईत् / सोम- वरुणयोः

Replace agneḥ with īt when soma-varuṇayoḥ follows. Inherited words (anuvṛtti) may add further conditions.

For the final of अग्नि is substituted the long ई when 1. सोम or 2. वरुण follow.
Thus अग्नीषोमौ, अग्नीवरुणौ ॥ The स is changed to ष by 8.3.82.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.