agni Ishvara rudra

Source: TW

The yajurveda is where the identification of the term Ishvara with rudra gets established. rudra the Ishvara can destroy the ritualists progeny & animals. Hence, he must pacify rudra.

TS:

सोऽनु॑ ध्यायति॒
स ई᳚श्व॒रो रु॒द्रो भू॒त्वा
प्र॒जां प॒शून् यज॑मानस्य॒ शम॑यितोः ।

Hence, down the line when in the ritual where agni is carried in the ass cart he is said to become the destructive Ishvara, TS:

स ई᳚श्व॒रः प्र॒जाः शु॒चा प्र॒दहः॑
शि॒वो भ॑व प्र॒जाभ्य॒ इत्या॑ह
प्र॒जाभ्य॑ ए॒वैनँ॑ शमयति ।

he is pacified by the incantation calling him shiva. This may be understood as agni having become his dual rudra and is pacified by the incantation “become shiva” for preventing him from burning the ritualist’s progeny.