Complex sentence: Mahishasur also gathered a large demon army to win for the heavens, in heaven, Indra learnt about the boon that Mahishasur got from Lord Brahma, he sent in a large heavenly army, Mahishasur came to the battlefield with his army as well with Indra's.
Split into simpler sentences: Mahishasur also gathered a large demon army to win for the heavens. In heaven, Indra learnt about the boon that Mahishasur got from Lord Brahma, he sent in a large heavenly army, Mahishasur came to the battlefield with his army as well with Indra's.