What does it mean to challenge a superior?

It is when a subordinate challenges a superior. The one who is a vassal takes the monarch in his place by killing or exiling him.

Subordination, ShogunicideIn addition to the rise of the Ise Nagashi clan in the Kanto region as the representative of a new era of daimyo, new powers began to emerge throughout Japan in which the subalterns were able to subjugate their superiors and even become the dominant power on one side. In China, the Izumo tutelary, on behalf of the Niko clan, usurped the power of the tutelary Kyogoku clan, and later its daimyo, Niko Norihisa, replaced the Kyogoku clan as the tutelary of Izumo. During his lifetime, the Niko clan expanded into a Chinese duo of eleven kingdoms, alongside the Ouchi clan's governor, Ouchi Yoshihide, who prospered from overseas trade.

In the process of the dual rivalry, the Angekoku people Mouri Gen took the attitude of a wallflower, gradually absorbing the power of the Niko and Ouchi, and later took advantage of the internal turmoil of the Ouchi clan, and in the Battle of Itsukushima knocked down the power of the Ouchi clan Tao Ching-hsien, and became the new hegemony.