Only Available after restoring the city
After getting the Klefki holding the Sanctum Key from The Klefki Sidequest go to the Subseven Sanctum in 7th Street and continue to the labyrinth.
After reaching the door on the left from the labyrinth you will reach this place.
There will be a Dark Material here, (there is also another one up the waterfall in Iolia Valley and a 3rd one there as well if you use rock climb) Field Read-Out 29 for the Holy Field, and the book will have some interesting lore, to the left you will see statues of arceus portrayed in many colors that is a hint for the Book Puzzle in the Library in Beryl Ward
Book Puzzle Edit
Go to the Library in Beryl Ward, one of the rooms in it will have different colored books interect with them in order:
A secret entrance will reveal itself inside having TM22 Solarbeam, Field Read-Out 35 for the New World Field, and a book with some interesting Lore as well.