Very recently (in Dec 2022) Dale Quarrington posted a fascinating in-depth post on the “Ten Kings of the Underworld” at the Koreantempleguide.com website. Here is a direct link to his post:
http://koreantempleguide.com/siwang-the-ten-kings-of-the-underworld-시왕/

Here is an excerpt from that post, along with one of the many beautiful images that illustrate the post:

Another tool of justice employed by King Yama is the Karma Mirror, which displays the sinner and their karma from their previous life. In “The Scripture on the Ten Kings,” it’s referred to directly as the “Mirror of Karma.” In Chinese, this is known as the “yejing,” while in Korean it’s known as the “eopgyeong.” Originally, the Karma Mirror might have been a precious object of the gods inhabiting the Buddhist heavens. It might also have been a precious object in Indra’s possession, where the karma of all the gods could be seen. In the context of the Buddhist underworld, it’s described in one text as follows: “In the hall inside the court of the Wise King [Guangming wang; namely, King Yama], there is a great mirror on a stand. The mirror of the Wise King is called, ‘Mirror of Purity, Rejecting Partiality.'”

Here is that link again:
Siwang — The Ten Kings of the Underworld: 시왕/