Complex sentence: Lord Indra took disguise of a crocodile and killed Karambh while Rambh successfully comepletely his penance and got a boon from Agni that he will obtain an extremely powerful son, who will rule over the three worlds in this universe.
Split into simpler sentences: Lord Indra took disguise of a crocodile and killed Karambh, while Rambh successfully completed his penance. This boon would allow him to obtain an extremely powerful son, who would rule over the three worlds in this universe.