Complex sentence: In a more sinister account given by Suetonius, Drusilla became pregnant with Caligula's child, and in a panic he disembowelled her, believing that the child of their union was divine and so represented a threat to his throne.
Split into simpler sentences: In a more sinister account given by Suetonius, Drusilla became pregnant with Caligula's child and that in a panic he disembowelled her. He did so in the belief that the child of their union was divine and so represented a threat to his throne.