originally this was not listed as a dimension book, but it was later designated as Dimension Book 15.

Official summary

Rifts® Secrets of the Atlanteans True Atlanteans are descendants from Earth’s past. The survivors of the sinking of Atlantis (really a dimensional mishap) and travelers of the Megaverse, wielders of Tattoo Magic and other lost mystic arts. Most people regard them as heroes, but are they? The Sunaj Assassins are mythic villains feared by all, yet they too are True Atlanteans who serve dark forces. For the first time, much of the story behind True Atlanteans and their secrets are revealed. True Atlanteans revisited. Optional Atlantean character creation tables including clan heritage and other factors. Secrets of the stone pyramids, different types/purposes and powers. Many new magic tattoos, magic spells, weapons and armor. Atlantean hideouts and secret communities across the Megaverse. The Sunaj Assassins, their secrets, history and plans for the future. Atlantean Monster Hunter O.C.C., Atlantean Defender O.C.C. and much more. And this is just the tip of what this book contains. Written by Carl Gleba. Additional text and ideas by Kevin Siembieda. Final page count and cost yet to be determined, but probably 160-192 pages – $20.95-24.95 retail – Cat. No. 890.

Dimension Books
Dimension Books

Rifts® Dimension Book™ 1: Wormwood®
Rifts® Dimension Book™ 2: Phase World®
Rifts® Dimension Book™ 3: Phase World® Sourcebook™
Rifts® Dimension Book™ 4: Skraypers™
Rifts® Dimension Book™ 5: Phase World®: Anvil Galaxy™
Rifts® Dimension Book™ 6: Phase World®: The Three Galaxies™
Rifts® Dimension Book™ 7: Megaverse® Builder
Rifts® Dimension Book™ 8: Naruni™ Wave Two
Rifts® Dimension Book™ 9: Phase World®: United Worlds of Warlock®
Rifts® Dimension Book™ 10: Hades - Pits of Hell™
Rifts® Dimension Book™ 11: Dyval - Hell Unleashed™
Rifts® Dimension Book™ 12: Phase World®: Dimensional Outbreak™
Rifts® Dimension Book™ 13: Phase World®: Fleets of the Three Galaxies™
Rifts® Dimension Book™ 14: Thundercloud Galaxy
Rifts® Dimension Book™ 15: Secrets of the Atlanteans
Phase World®: Heroes of the Megaverse®
Guide to the Three Galaxies
The Grand Paladins
Other Books: Sourcebooks * Conversion Books * Dimension Books