Complex sentence: The progress of Bhandi's march is not known for certain, but there can be no doubt that Shashanka continued to rule his empire vigorously, which included northern Orissa and southern deltaic regions of Bengal.
Split into simpler sentences: The progress of Bhandi's march is not known. But there can be no doubt that Shashanka continued to rule his empire vigorously, which included northern Orissa and southern deltaic regions of Bengal.