Tooth enamel is the hardest part of the body but it still can crack, chip and break. When it has a small crack, it may cause crack tooth syndrome which involves sharp pain when biting on it. When the crack travels deeper and touches the nerve inside the tooth, it will cause severe toothache, often it's too painful to sleep and ends up with dental emergency.
In this case, the crack on the distal was too deep for the nerve to survive and a root canal treatment was unavoidable. Finally the root canal treatment was completed successfully with a dental operating microscope. A final crown restoration will be needed in near future.