The Barrier (結界, kekkai) is a net-like energy barrier, established by the Great King Enma, that divides Demon World from Human World, resting in the pseudo space. Only demons below C Class can pass through it. A more powerful net was going to be made, but net energy was the energy most easily spread to cover the distance in the pseudo space.
History[]
Before Yu Yu Hakusho[]
Knowing that they could not defeat the S Class demons living in Demon World, the leaders of Spirit World raised a barrier around the pseudo space around Demon World in the form of a net to stop both A Class and S Class demons from entering the Human World. Due to limited resources, this barrier could not extend to blocking B Class demons from entering the Human World.
Chapter Black Saga / Three Kings Saga[]
It was cut down by Kazuma Kuwabara, and his newly-developed Dimension Sword, while chasing Shinobu Sensui along with Kurama and Hiei, to Demon World during the events of the Chapter Black Saga. Later on, the Barrier was rebuilt by the Special Defense Force of Spirit World. This was until the outcome of Three Kings Saga, where said barrier was removed and demons were free to travel between Human World and Demon World, but some S Class and A Class subordinates which participated in the Demon World Tournament had the task of patrolling the entrance between both realms in order to control any possible frenzy, mishap or if any demon went rogue.
Two Shots / All or Nothing[]
During the epilogue of the manga and in Two Shots / All or Nothing 2018 OVA, Shunjun appeared before Yusuke. He told the news to the young Spirit Detective that the Gate of Judgement has been seized and both Koenma and Botan have been taken hostage with the terrorists demanding the removal of demons from the Human World and the restoration the Barrier. After Shunjun explained the situation, Yusuke called Demon World ruler Enki and requested him to gather his old friends. As Team Urameshi was gathered, they left their bodies and went into the Spirit World. Ultimately, Yusuke ended up succeeding with the mission and both Spirit World and Human World were sound and safe once more.