Imbrium Basin 20,000 B.C.
|Overcoming the Imbrium Basin 20,000 B.C.||Beat the Imbrium Basin 20,000 B.C.||x100|
Charol Plains 
Skills: Imbrium Basin 20,000 B.C. (Hidden Boss)
|Starts battle with Sacred Water Stance|
|HP stopper at 50% HP|
|-||Goldilocks||Inflict poison (fixed 1500 poison damge) on all enemies + Deploy Sacred Water Stance|
|-||NA-19999A||Water type magic attack on all enemies|
|-||Quantum Hall||Fixed 2000 damage on all enemies + inflict Pain (fixed 1500 pain damge) on all enemies|
|-||Degeneracy||Burn 1/3 of current MP + inflict Bind and Sleep on all enemies|
|Push a opponent to reserve|
Water type magic attack on all enemies
|-||Superconductor||Random water piercing attack x3 + inflict Seal|
How to find Imbrium Basin 20,000 B.C.
To unlock this battle, you will need the "Spirit Detector". This can be found in Spirit Sanctuary after completing the Zerberiya: Sword and Wings Mythos: Song of Sword and Wings of Lost Paradise Mythos, chapter 8. Go back to the boss room and check the hidden room on the far right (now accessible due to collapsing pillar).
Imbrium Basin 20,000 B.C. itself can be found in its own area in Charol Plains. Entering Charol Plains with the Spirit Detector item will cause the entrance to this area to appear at the far western end of Charol Plains, next to the entrance to Vega Forest.