These portals are like purple glowing holes in appearance that show up in the ground. They directly connect with Hell, where the Oni exist. Usually, the Oni don't leave Hell due to having no means to do so, but if rebellious Onis see the chance of going through a Hell Hole they will take it to go to the mortal realm and cause trouble, usually to have fun by beating up people, raping women and destroying towns. Not all Onis are like this, since some are nicer individuals that dutifully do their jobs. However, the ones that are seen coming out of Hell Holes, are the cruel ones that go against the Oni rules, so they are expected to be trouble-makers.
The Hell Holes can appear in any place in JAPAN at any point, but it doesn't happen to often. When it does happen, diviners of the Houjou House are in charge of disposing the Oni and closing the Hell Hole. A tecnique created through Yin Yang Arts in the year GI0460 to block these portals and avoid more Oni from coming out. Samurais and different warriors also fight the Oni if Diviners are not around though.
The only place where the Hojou House doesn't venture is the province known as Shikoku, the "Death Country". This huge area is a huge place where undesires like bandits or cursed humans are thrown. It is plagued with Hell Holes and those inside are trapped by huge walls to avoid their escape, being forced to fight Oni everyday to survive. The Takuga House is notorious for being founded in this area.
Just like Oni can use Hell Holes to get to the mortal realm, humans can use the Hell Holes to venture into Hell. However, this would be usually for almost anyone and has little to gain. The only known human to have done such thing was the Emperor Fujiwara Ishimaru.