Which goddess became the ruler of the underworld Yomi?

The goddess who became the ruler of the underworld Yomi was Izanami.

Izanami dies after giving birth to the fire deity Kagutsuchi, whose flames fatally injure her. After her death, Izanagi travels to Yomi, the land of the dead, hoping to bring her back. Izanami tells him that she has already eaten Yomi’s food and can no longer simply return to the world of the living.

When Izanagi sees her decayed corpse, he flees in horror. Izanami, enraged by his rejection, pursues him and becomes associated with the dead and the power of Yomi. Their confrontation ends with Izanagi sealing the entrance with a huge boulder, after which they declare a permanent separation between life and death.

The story is often compared with underworld-return myths such as Orpheus and Eurydice, but its details are distinctly Japanese. Izanami is not the same figure as Amaterasu, the sun kami, while Izanagi remains a creator deity who later purifies himself and produces Amaterasu, Tsukuyomi, and Susanoo.