The mountain thus looked like a mass of dark clouds charged with lightning. O Brahmana, the fire spread, and consumed the lions, elephants and other creatures that were on the mountain. Then Indra extinguished that fire by pouring down heavy showers.

In plain words: The mountain looked like a dark storm cloud full of lightning. Fire spread and burned the lions, elephants and other animals living on it, until Indra put it out with heavy rain.