Sinus Aestuum 20,000 B.C.
|Overcoming the Sinus Aestuum 20,000 B.C.||Beat the Sinus Aestuum 20,000 B.C.||x100|
Vasu Mountains 
Skills: Sinus Aestuum 20,000 B.C. (Hidden Boss)
|Starts battle with Raging Fire Stance|
|HP stopper at 50% HP and Deploys Raging Fire Stance|
|-||Solar Storm||Fixed 2000 damage on all enemies + inflict 1500 damage Poison|
|-||Heliosphere||Raging Fire Stance + All enemies Fire RES Down L (3 turns)Deploy|
|-||Bow Shock||Self PWR/INT Up S (2 turns) + inflict 1500 damage Poison to all enemies|
|-||FRB x2||Inflicts damage to one enemy equal to 90% of its current HP|
|-||Blazar||Fire-type slash attack on one enemy|
|-||Quasar||Fire-type magic attack on all enemies|
How to find Sinus Aestuum 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).
Sinus Aestuum 20,000 B.C. itself can be found in its own area in Vasu Mountains. Once you have the Spirit Detector, you can take the path in Vasu Mountains from Ratle to Nadara Volcano and you can't miss detecting and unlocking it, located at the westernmost end of the path directly above the path coming from Ratle.
- Melissa's Flash Strike Stance can override the initial Fire Zone for a turn 1 burst strat. Aim to rush to the AF stopper, then swap her out afterwards to establish Water Zone.
- Anabel (Another Style) can establish Water Zone on demand to counter its random Heliospheres, without needing to spend turns resetting a VC.
- Make sure everyone has at least 3500 HP, otherwise Solar Storm will kill them.
- Alternatively, characters like Nagi (Extra Style) or Levia (Another Style) can apply status immunity to the party, protecting you from the poison damage. If using Levia AS, the poison inflicted on your party from Levia Buster will not be overwritten by the poison from Solar Storm, causing your party to take significantly less poison damage at the end of the turn.