Complex sentence: Those British who were captured were placed in a punishment cell which became infamous as the Black Hole of Calcutta and, in the stifling summer heat, it is alleged 123 of the 146 prisoners died as a result of suffocation or heat stroke.
Split into simpler sentences: Those British who were captured were placed in a punishment cell which became infamous as the Black Hole of Calcutta. In stifling summer heat, it was alleged that 123 of the 146 prisoners died as a result of suffocation or heat stroke.