In Japan, there is still immense stigma that surrounds tattoos. This stems from the use of tattoos to mark prisoners and criminals in ancient times, and also from the use of tattoos by the Japanese mafia, called the 'Yakuza'. A lot of businesses like gyms, saunas, etc., still ban customers with tattoos even today.
Oni are believed to be supernatural creatures who are the guardians of hell. They are the ones that torture and punish the souls sent there. Amongst a lot of different beliefs, Oni are also said to be cannibals, the ones that punish sinners, and the cause for disease.
Contrary to popular belief, Oni are not only evil. Some are protectors, known to bring good health, peace, and protection from evil spirits. One such story is of a monk that starved himself to turn into an Oni and protect his temple.
The Oni mask is often the centerpiece of Horimono (full body tattoo). The Horimono can take years to complete, and is normally done in weekly sittings. This makes it extremely expensive. Traditionally created by almost carving the tattoo, the sheer pain, time, and effort of this tattoo signifies the person's dedication and strength.