Of all the Vedic sacrifices, Somayāga is, perhaps, one of the most complicated. A separate altar called mahāvedi is to be erected for the conduct of the Soma group of sacrifices. Since the yajamāna or sacrificer has already a vihāra (an old and permanent shed containing the three fires gārhapatya, āhavanīya and dakṣiṇāgni), the fire for the new sacrifices has to be carried from the vihāra to the mahāvedi. This is done as a part of the whole ritual.
Taking out the agni (fire) from the old āhavanīya in the vihāra, as also the soma stalks from which soma juice is to be extracted for the Somayāga in a ceremonial procession is called ‘agnīṣoma-praṇayana.’ This is done on the day before the sutyā (day of extraction of soma juice). The adhvaryu carries the fire on clay, the yajamāna and patnī along with their sons and grandsons following him, one behind the other, in a long procession. Soma stalks as also all the vessels needed in the sacrifice are brought in a cart and deposited in their respective places. The fire is deposited and kindled in his pit by the āgnīdhra priest.