From the outside, it looks like just another cave in a mountain on an island in the ruined future, but inside it looks a lot like the Abyssal Temple. "Manoria Abbey" plays, but investigating a sparkle prompts an odd eyeball to rise out of the lava, surround itself with flames, and attack! Not only is it a boss, Son of Sun even has the special alternate boss music.

Don't attack the eyeball directly, as it counters with Flare and is immune to essentially all forms of attack anyway. The flames absorb all elements and barely take any damage from other attacks either, and of the five flames, four will counter with a Fire-based attack:

The fifth is, of course, special, and attacking it makes a "correct" noise and damages the eyeball. The correct flame to hit is chosen at random, and the eyeball occasionally uses ルーレットシャッフル (Roulette Shuffle) to randomly switch which is the correct one. Furthermore, attacking all at once provokes counterattacks from each foe that was hit (including the eyeball), except that the correct target will do nothing and the eyeball will NOT take damage.

There are a few tricks to make this battle a lot easier. If the Magus has learned ★Black Hole, it can permanently remove up to two flames from the battle, improving your odds of guessing the right one after the shuffling (if he swallows the current correct target, you'll have to wait for a shuffle before you can do anything). More importantly, virtually all the attacks the enemies use are Fire-based, so characters wearing Ruby Armor take minimal damage, and those with a Red Vest or Red Plate will actually recover health. Furthermore, since the correct flame doesn't react to multi-target attacks, you can tell which one it is if you're paying attention and don't mind provoking counters from everything else.

Ayla can steal a rare Black Plate from the eyeball and Elixirs from the flames (stealing also triggers the counterattacks, but it's worth it).

After enough hits, a message appears...

...and soon after, the flames die out and the battle ends. The previous music resumes, and a bridge, which the eyeball quickly scurries across, appears over the lava. The eyeball retreats to the back of the temple and disappears, leaving behind a large grey stone.

Marle: Isn't this the Solar Stone
from the Ancient Era?
But it's stopped shining...
If we keep it exposed to light again
it might be able to regain its light.
Hmm. But it's sure gonna need
a reeaally long time.

Marle: Isn't this the ancient Sun
Stone?
But it no longer glows...
Maybe it will regain its glow if we
keep it in the sunlight!
Might take a while, though.

Lucca: This is the Solar Stone
of the Ancient Era?
It's lost its light over this long time...
If we expose it to light once again,
it might be able to regain its energy.
But it's probably going to need a
mind-bogglingly long time.

Lucca: This is the ancient Sun
Stone?
It doesn't seem to have an ounce of
energy left in it.
We could probably recharge it with
sunlight, but that would take aeons.

Frog: This is that Solar Stone
thing from the Ancient Era?
It's stopped shining entirely.
Keeping it exposed to light
again might restore it.
But such time it would need
to store up the sun's power.

Frog: 'Tis the ancient Sun Stone!
But it lacketh luster.
Perhaps the sun can rekindle it!
But how long will that take?

Frog's last line is more a statement of awe than a question, so no question mark and no question phrasing.

Robo: This IS the Solar Stone of the
Ancient Era? It SEEMS that it has
lost its light over this long time.
However, if we continue exposing
it to light once again, it may BE
able to regain its energy.
By my conjecture, it WILL likely
require at least 65 million years.

Robo: This is the ancient Sun
Stone?
The years must have robbed it of its
sparkle.
But if we expose it to sunlight for
approximately 65.2167 million years
we just might be able to recharge it.

Nonsense fabricated decimal in the NA version.

Magus: Oh? It's the Solar Stone.
So, it's lost its light over all this time.
But if we continue to expose it to the
sun's light once more, it should be
able to regain its energy.
Likely needs at least 65 million years.

Magus: So this is the Sun Stone.
Its power has been lost over time.
With the sun's energy, it could be
recharged.
But it'll take over 65 million years.

Over all the millions of years the Sylbird can cross, this one island remains essentially unchanged. Regardless of the era, "Manoria Abbey" plays, and a strong beam of sunlight shines inside.

Marle: The sun's light...
This has got to be the place
most loved by the sun.

Marle: Wow, sunlight!
This must be the sun's favorite spot.

Lucca: It's never dark in here.

Frog: Here...?
It's warm, for sure.

Frog: This be the spot.
'Tis warm.

Robo: So HERE is a place where
sunlight hits 24 hours a day.

Robo: The sun shines 24 hrs.
a day here.

Ayla: Ohh, toasty!
Sun hitting well!

Ayla: Is cozy!
Sunshine good!

Magus: The Shrine of Light...
They say the sun's light has kept
shining here since the distant past...

Magus: The Sun Keep...
Where the sun never sets.

They leave the stone in the beam of light.

Marle: Now, let's hop aboard
Epoch!

Lucca: Okay... well then, let's try going
in the Sylbird.

Lucca: Well, back to
Epoch.

Frog: ...wait, how long have we got to
let it sleep!?

Frog: How long shall we leave
it hither?

Another attempt at fancy language gone wrong. "Hither" is not interchangeable with "here" as it includes a sense of movement (toward here).

Robo: NOW we need only go to MY
era in the Sylbird.

Robo: Let's return to my era in
Epoch.

Ayla: ......
Not change yet.

Ayla: ...
No change.

Magus: Now, let us go.

This ought to be long enough... but the stone is missing.

Marle: It-it's gone!
The Darkness Stone's gone!!
But it looks like quite a while's
passed since it disappeared.
Let's try going to a different era.

Marle: It's not here!
The Moon Stone is gone!
It's been gone for ages.
Let's check out another time.

Lucca: It-it's gone!!
The Darkness Stone is gone!
But it looks like a quite a while's
passed since it disappeared...
Let's try going to a different era.

Lucca: It's gone!!
The Moon Stone is gone!
It looks like it's been gone for ages.
Let's try another time period.

Frog: It-it's gone!!
The Darkness Stone...
But it looks like quite a while's
passed since it disappeared.
Let's try going to a different era.

Frog: The Moon Stone, 'tis gone!
Yet it hast not been taken recently.
Let us return in time.

Robo: It, it is NOT here!!
The Darkness Stone is NOWHERE...
HOWEVER, it SEEMS that considerable
time has passed since it disappeared.
Let us try going back a bit in eras.

Robo: It isn't here!!
The Moon Stone isn't anywhere.
It wasn't taken recently, though.
Let's go back in time a bit.

Ayla: ......
Gone... stone gone?
But much dust.
Someone took away
way long ago for sure.

Ayla: ...
Gone...rock not here?
But plenty dust.
Someone take long time ago.

Magus: ......
The Darkness Stone is gone...
............
It seems around 1300 years have
passed since it disappeared.
We're going to the Modern Age.

Magus: ...
The Moon Stone is gone.
...
Seems it's been gone for 1300 years
or so.
Let's get back to the present.

Did he just... SENSE how long it was gone...? Seems like his magic's more versatile than it appeared.

The stone is present in all previous eras, getting gradually brighter, up to A.D. 600 (don't miss the Power Capsule then!), but missing in this one.

Actually, it's frankly amazing that it was left alone for that long, especially through the Zeal era.

Marle: It's not here either...
I wonder if someone from this
era took it away?

Marle: Not here either...
Did someone from this age take it?

Lucca: It's not here either...
I wonder if someone from this
era carried it off?

Lucca: It isn't here either.
I wonder if someone from this time
period has taken it?

Frog: Not here either...
Did someone from this era take
it away?

Frog: Not here either...
Hast someone from this time taken
it?

Robo: It seems NOT to be here either.
Might someone of this era
have removed it?

Robo: Doesn't look like it's here
either.
Has someone from this era taken it?

Ayla: Not here either.
Someone took away for sure.
We search!!

Ayla: Not here too.
Someone take!
We find!

Magus: It doesn't seem to have been
that long since it was removed.
We're searching the area.

Magus: It hasn't been gone for long.
Let's take a look around.

Not far to the northeast, the party spots a suspicious light shining from the town leader's house...

...but he's not talking, and no one else knows anything. But maybe if we do something to change history...

...there's a woman in the elder's house who might be able to help...

Since this is where we got some last time, let's try asking the head cook...

Maybe in the Modern Age, then?

Sorry, NA version, but your clue is broken.

There's a restaurant in town that might have that sort of thing...

It's obviously overpriced, but he won't bargain, and at this point in the game it should be easily affordable.

Sure, you can go for the 100 G profit, but that's nothing, and besides, the whole point of this is to try making the town leader in the modern era a nicer person. Choose ただであげる (Give it for free).

That sounds promising. Time to check on the Modern Age yet again...

Well, the kids are crowded around the father instead of avoiding him. That looks like a good sign.

The party regains the stone.

Though that still leaves the question of who brought it here, and why.

Having replaced the stone in the shrine, the party heads once again to the future. The stone is still here this time, and now glows brightly.

Lucca: Amazing...
With this much energy, I should be
able to make a powerful weapon!
Let's go to my house right away!!

Lucca: Amazing!
With THIS much energy I can make a
powerful weapon!
Let's go back to my house now!

Marle: Eeek! AMAZING!
Hey, Lucca could use this
to make something, right?
Let's try showing her!

Marle: Acck! This is amazing!
Lucca can surely put this to good
use!
Let's show it to her!

Frog: Th-this's amazing...
Maybe Lucca could use this
and make something...
Are we going to show her?

Frog: T, 'tis amazing...!
What might Lucca create with it?
She must see this!

Robo: AMAZING energy readings.
Miss Lucca MAY be able to
use THIS...
LET US call her.

Robo: My sensors read high energy
levels.
Lucca may be able to use this.
Let us show it to her.

Ayla: Wow! Sun!!
Oh! Lucca maybe can use
this. We call her!

Ayla: Wow!
The sun!
Lucca maybe use?
Bring Lucca here!

Magus: So the Solar Stone has
regained its radiance...
but how to use this energy...?

Magus: The Sun Stone has regained
its luster but...
How might we harness its energy?

Lucca: It sure is a Solar Stone...
With this much energy, I should be
able to make a powerful weapon!
Let's go to my house right away!!

Lucca: This really is the Sun Stone!
With this energy, I can make an
incredible weapon!
Let's get back to my house quickly!

Lucca sets to work with one of her machines, and "Merry Specchio" plays.

ルッカ「太陽石エネルギーを
　　あつかいやすいパワーにせいぎょし…

Lucca: Regulate the Solar Stone energy
into a power that's easier to handle...

Lucca: I'll modify the Sun Stone to
extract its energy...

ルッカ「しんくうカートリッジに
　　パッケージング……。

Lucca: Package it into a vacuum cartridge...

Lucca: ...then, we'll just vacuum
pack it into a cartridge...

Lucca fiddles with the machine, which responds by making some odd lights and noises.

Lucca: ...and it's finished!

She takes something out of the machine, does a little victory dance, and waves a gun around.

The Miracle Shot is by far the most powerful of Lucca's guns, though it's also highly unpredictable. See the items page for details.

Lucca: Sometimes I amaze myself!

Taban: Look, Lucca!
I made this with a little determination too.
Borrowed a smidgen of the Solar Stone.

TABAN: Take a look Lucca!
I borrowed a bit of the Sun Stone to
create something nifty!

Sidequest clear! Rewards: Black Plate (steal), Power Capsule, Miracle Shot, Solar Glasses. Also, there are additional rewards for clearing both this quest and the Prismatic Shell quest (in either order).

Though it was more of a means to an end, reversing the attitude of Palepoli's mayor seems like it could end up being historically significant.