The Forbidden Lands is a near empty dimension for those who wish to have more land in their world.
Blast a Diamond to created a Sinned Diamond. From there, Blast That, to Create a Mysterious Gem. You have two options. Trade it with a Librarian, or Smoke the Mysterious Gem. Both Give you The Mysterious Alter.
You need to brew A Sinned Diamond with Mossy Cobblestone in order to create the Igniter.
Place the Mysterious Alter in a Crafting Table to get 20 Last Hope blocks. That is the Frame For The Portal. Now, Enter As You Wish.
The Forbidden Lands Team
