What did Fujiwara Michinaga do when he left the Heian court in the eleventh century?

He joined a Buddhist monastery but continue to run Japan.

Fujiwara Michinaga was the most prominent of the Fujiwara regents, amid whose reign the Majestic capital in Kyōto accomplished its most prominent quality, and the Fujiwara family, which ruled the Japanese court somewhere in the range of 857 and 1160, achieved the apogee of its rule.

A progression of heads began to resign to a monastery early in life, and put their young children on the throne to run the nation from behind the curtains.