Complex sentence: Mastani also gives birth to a son, whom Mastani and Bajirao name Krishna Rao, but when Krishnaji Bhatt, the head Brahmin priest, refuses to conduct the "naamkaran" (Hindu christening ceremony) and he is deemed illegitimate, Bajirao names him Shamsher Bahadur.
Split into simpler sentences: Mastani also gives birth to a son, whom Mastani and Bajirao name Krishna Rao. However, when Krishna Bhatt, the head Brahmin priest, refuses to conduct the "naamkaran" (the Hindu christening ceremony) on the grounds that Krishna Rao is deemed illegitimate, Bajirao renames him Shamsher Bahadur.