Lacus Solitudinis 20,000 B.C.
|Overcoming the Lacus Solitudinis 20,000 B.C.||Beat the Lacus Solitudinis 20,000 B.C.||x100|
Chasselas Crystal Region - South 
Skills: Lacus Solitudinis 20,000 B.C. (Hidden Boss)
|Starts battle with Torn Earth Stance|
|HP stopper at 50% HP and deploys Torn Earth Stance|
|-||Lonsdaleite||Deploy Torn Earth Stance and damage nullification shield to self (30 times)|
|-||Void Galaxy||Earth type magic attack on all enemies|
|-||Magnetar||Fixed 1000 damage on all enemies, increase by 1000 damage on subsequent use|
|-||Dark Nebula||Earth type blunt attack on all enemies + PWR down (2 turns)|
|-||Pulsar||Earth type slash attack on one enemy+ INT debuff (3 turns)|
|-||Amorphous||Restore 10m HP to self + Physical Res and Type Res UP L (2 turns)|
How to find Lacus Solitudinis 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).
Lacus Solitudinis 20,000 B.C. itself can be found in its own area in the Chasselas Crystal Region - South. Once you have the Spirit Detector, you can ride over from the Winged People's Village and it will be a short walk directly north.