So basically the place that AHIH made at a certain point in time.
Wishing a more magical and fantastic world, AHIH eventually made a new world. This one is even more magical, but compared to the combat space (which became a semi-real universe with the Causal Membrane as the Proto-IL), this world is accessible from both sides.
This is the "World Within a Book".
An anything-goes world where nobody dies, nobody goes hungry, magic is easy, disease is just a story, animals can talk, and wearing a witch hat can make you able to use witch-related spells. It doesn't really need to make sense, since it's a fake world anyway.
The only real people are the ones who enter. They can leave if they find an exit, but some choose to stay because immortality and an easy life are things they desire. The world expands so it won't get overcrowded, and the book itself seems to have unlimited pages where the story of the world and every person keeps going.
The stars are painted. You can fly and touch them. The world extends underground without limit for those who like digging.
There are no other planets. The moon and sun are also strange—sometimes there can be up to thirty small suns, sometimes a smiling sun wearing sunglasses.
There are rules, made to keep things friendly and ensure people treat each other well.
There are also designated places with more adult themes. People eventually get bored of constant sunshine and safety, so violence or crime can exist there, but only if they repeatedly choose it. Once a day, they are pulled out and asked if they still want to stay.
There are gold cauldrons at the end of rainbows.
Time flows at 1000:1, meaning time inside the book moves much slower. One girl stayed there for three thousand years.
Almost anything can happen there. It only becomes difficult if someone wants it to be difficult. The same girl eventually forgot she came from World Zero. Another academy student entered the book as well; AHIH met her during a moment of rest and convinced her to return because her friends were worried. She had been missing three months in real time, but inside she experienced roughly 250 years.
It is basically a vacation place for AHIH and others. AHIH can naturally find it at any point in time just by waiting. Others may stumble into it. Right now, the Heroic Academy of the Salvation Church has it hidden deep in their library, placed there at True Year Zero.
AHIH used it to plan ahead and rest, since young AHIH before ascension carried the weight of the world. Even with causality on its side, mistakes were still possible, and a war with heavy casualties that could have been avoided might still happen.
This was made by the Very Old AHIH, near the point where it was almost indistinguishable from the Main Body Lehatot.
It exists so the Younger AHIHs can continue their roles.
It was also made so that anybody that attempts to monopolize it that is not AHIH or a Lehatot will find that at one point in time, it simply disappears.