Use the Dimensional Gate in Atlantis to reach the Obsidian Turtle Pool. Kill 30 Obsidian Warriors and 20 White Bison, then speak to Minogame.

Show Quest Info

Introduction

Roshi and Minogame may be the only ones left in Obsidian Tortoise Pool that are still relatively sane.
Roshi doesn't know what is driving the Obsidian Turtle to the warpath, but crippling his forces should by the time you need to investigate further. Minogame may be able to give you more information.

NPC Dialog

Some strange has been going on here recently. The Obsidian Turtle, oh, he's just gone completely mad. There's no telling why, but this isn't like him at all, no. His forces are gathering already! That's why I wrote to Muto, yes; I think this may be a threat even to Atlantis. You should beat some sense into his troops, and then go speak to Minogame. He'll be able to tell you more!

Quest Completion

Oh, so you're the one who has come from Atlantis to help. I suppose we could have done worse...