असौ यज्ञ: this sacrifice, मन्त्रवत् in accordance with mantras, यथान्यायम् properly, सम्प्रवर्तते was being performed, आकाशे in the sky, भयानक: causing fear, महान् शब्द: great sound, प्रादुरासीत् emerged.

While the sacrifice was being performed in accordance with mantras, a great sound causing fear emerged from the sky.