The Buddhist deity who rules the underworld and judges the dead is Yama.
In Buddhist mythology, Yama is a dharmapala who presides over hellish realms and evaluates the deeds of the deceased. The figure derives from the Vedic and Hindu god Yama, but Buddhist traditions developed their own stories and roles for him. His judgment is connected to karma and the continuing cycle of death and rebirth.
In the Pali Canon, Yama questions wrongdoers about whether they recognized warnings such as birth, aging, sickness, worldly consequences, and death. In Tibetan Buddhism he appears as Yama Dharmaraja, also called Kalarupa or Shinje, and is depicted as a powerful guardian of practice. In East Asia he became Yanluo Wang in China and Enma in Japan.
A common mix-up is identifying Yama with Kshitigarbha. Kshitigarbha is a bodhisattva who vows to aid beings in the hells, whereas Yama is principally the judge or ruler associated with the underworld.