Afterlife

The Afterlife or the Underworld is the dimension or plain of existence where all living beings, plants, and animals go after their death, and is also where the higher deities of the universe reside. Though it has yet to be seen officially in the series itself, it has been mentioned and referenced several times by various characters, most notably Lala whenever she slips into Chuunibyou.

Current Inhabitants

 * Anubis
 * Dullahan
 * Grim Reaper
 * Hel
 * Shinigami

Former Inhabitants

 * Abyss
 * Death
 * Halifa
 * Lala

Locations

 * Sanzu River

Ways to Access the Afterlife
For most living beings, there is really only one way to enter the afterlife and that is their death. As Lala explains, when a living being dies, only its body dies, but its soul is immortal and goes to the afterlife after its death. Apart from that, Dullahans and other psychopomps like Grim Reapers can walk between the worlds as "Herald of Death" and enter and leave the afterlife at any time of their own free will.

Ways to Leave the Afterlife
Usually death is final, but many times throughout the series, Kimihito has shown the unusual strength of facing death, despite the numerous near death experiences he had. As Lala explains, that will is based on his caring for others, not on fear of his own death.

Apart from that, there is also the possibility for a living being to return as a Ghost in order to come into contact with the living, which is independent of whether they were Human or Liminal during their lifetime. However, the exact possibility with which one can become a ghost is unknown and it seems to be a very rare event anyway, presumably because most people accept their death and therefore do not want to return to the world of the living.

Wladislaus Drakulya appeared as a ghost after his death and took control of his daughter in order to use her as a means for his own ends. However, according to Lala, the dead have no right to do violence to the living, so she used her power as a Dullahan to sever Wladislaus' connection to Curie, thereby severing his connection with the living world.

It's not known whether animals can also return as ghosts, but this presumably happens very rarely, as many animals don't have a higher level of consciousness and therefore do not have the will to return to the world of the living.

Trivia

 * As Lala reveals in Monster Musume: Monster Girls on the Job!, it's impossible to sleep in the afterlife. This makes sense since most residents of the afterlife are dead and therefore no longer need or feel the need for sleep.
 * Whenever Kimihito wakes up from a near-death experience, he cannot remember his afterlife experiences, which is why he hasn't been able to tell anyone about them until now.