I hope you get good sleep along the way coz this might take long
There are direct trains but no direct flights for this journey. Chennai (Tamil Nadu) to Varanasi (Uttar Pradesh) distance is 1860 km and you can get there by flight + flight in just over 7 hours and 15 minutes or by train fares starting from ₹ 633.
The heat there will make you sweat, but it's surely bearable.