The Lunar Cultivator

Main character is named Tango Fengii, he was in the time period of 3472. His mission was to help stop a tyrant and he was making great strides with the resistance. Then he was found and "dealt".
Unfortunately he failed and his future was bleak the tyrant had caused the end of galactic civilizations, after his fall various factions fought for power and ended up putting everyone back into the dark ages.