Enma Daio (also known as Great King Enma) is a frightening and demonic entity from ancient Buddhist mythology as well as Far Eastern beliefs, due to such a wide-reaching influence his appearance and traits may differ from region to region but in general he is seen as one of the thirteen judges of Hell (though it is somewhat misleading as he is most active in the realm of Meido, where souls are sent on many trials to decide if they may pass to Heaven (Tengoku) or Hell (Jigoku) - however, he is the Judge who is responsible for ensuring every soul that ends up in Jigoku is given their proper amount of torture and stay - which can vary from centuries to measures of time so vast it is said to be beyond understanding).
As such he is one of the most important deities in relation to humanity and while a strict, extremely vicious being he is also the alter-ego of the god known as Jizō Bosats - who is beloved for his role as guardian of the Underworld, protector of children and god of travellers.
It is well known that Jizō Bosats made a solemn oath to never obtain full deity-status until every soul was saved from the torments of Hell and thus Enma, as feared and powerful a demon as he is, actually holds a more benevolent goal under his fearsome exterior and deeply wishes to see humans saved from Hell
This is why Enma and his fellow judges are so cruel and demanding in regards to the tasks they put forth and while many souls will inevitably fail these tests and be damned to Hell it is Enma who is most concerned with giving these souls as many chances as possible to avoid further suffering.
This concept of Enma being both a tyrannical demon and a loving deity is common and all the judges of Hell share Enma's dualistic nature as alter-egoes of gods - thus they are all working, in some way, as divine retribution and learning (albeit a brutal form of said learning).
Enma is served by all manner of demons and Oni, the most fearsome of which are Gozu and Mezu - gigantic and terrible Oni with the heads of an ox and horse, who act both as guardians of Hell and as enforcers of Enma who shall pursue any who manage to escape Hell and bring them back for proper punishment and trials.
Also as an aspect of a deity Enma has unfathomable power in his own right and continually writes down the good and bad deeds of mortals upon a large scroll, using each deed as a way to judge them - he may also (depending on the story) use magic mirrors or other devices to gaze into a mortal's life and see all the good and bad they have done, it is then mighty Enma who decides what realm the mortal is to be assigned to - there is no real limit to what Enma can do in this regard and he is said to be able to grant a soul entrance to Heaven or damnation to Hell, though often he will instead bring them to one of the other realms (such as that of humanity or animals): as always it is ultimately decided by the collective good or evil one has commited in life, the most evil of souls will be cast to Hell immediatedly while those that are neither truly good nor truly evil may undergo further trials before any reincarnation, all at the whim of Enma.
Enma is also said by many tales to be one of the last judges capable of sparing a mortal from Hell if their living relations follow certain prayers and rituals, for this reason the 49th day after a person's death is considered especially important in traditional mourning rituals.
While a caring deity at heart Enma is also said to dictate exceptionally brutal torture and mutilation upon people even for minor misdeeds, for example children would have their tongues ripped out for lying and even killing lowly insects may serve one a lifetime of torture in one of the many "lesser" Hells.
Souls of all beings (both good and wicked) are also subjected to brutal treatment by Oni under Enma's command, who force them to climb mountains and cross rivers in order to even reach the Judges who will dictate their final fate - though family prayers and the good karma of a soul's previous life may ease some of these sufferings, only the most pure of souls will be taken to Heaven and only the most wicked will be cast instantly to Hell : thus a vast majority of souls will undergo some form of torture under this very strict system of law.