Because the Dead Area was a map that was released in a later expansion, it hadn't been completely explored by the time Marvin transmigrated.
As for the Secret Garden, many player guilds explored it and gathered enough information, but because of the special nature of the instance, Marvin himself hadn't personally experienced it. He had obtained some secondhand information, but it was difficult to say whether it was all accurate.
From what he remembered, the origin of the Secret Garden was related to a Wizard's school of magic from an ancient era.
That school of magic was called the Mikenshi school. Mikenshi Wizards were focused on cultivating Magic Medicines, and in their golden age, their organization's branches covered every corner of the Dead Area. Of course, much had changed over time in the Dead Area.
