The Land of the Sun was invaded by Lucius Zogratis' paladins, who managed to defeat Ichika Yami and four other Ryuzen. Asta helped by rescuing Sister Lily on the Zetten after completing the training. The fight goes on, though Black clover 351 takes the reader into the past.
As he battles the last paladin, Yosuga travels through the depths of his memories. In fact, the strongest of the Ryuzen remembers contesting the title of Shogun of the Land of the Sun with Ryuya Ryudo in his youth.
In Chapter 351 of Black Clover this is toldheated rivalry between Yosuga and Ryuya. Even as youngsters, they were considered the strongest fighters in the Land of the Sun and possessed the most Yoryoku. However, in Black Clover 341, we were told that Ryuya doesn't have magic powers, but instead has a special eye. So what happened to the Shogun?
Once upon a time there was a plague that swept the land of the sun, and the only way to save its inhabitants was to find a healing spring through them Tengentsu. The only ones who could acquire this magic eye were Yosuga and Ryuya, but in return they had to sacrifice all of their Yoryoku resources. While Yosuga hesitated, Ryuya accepted without hesitation.
Ryuya has therefore lost all his magical powers to acquire a magic eye capable of saving the people of the Land of the Sun.