On the way to the mountain, Cecil can kill with ease. Once here, however, most enemies are Undead and highly resistant to his Dark Sword, so you'll need magic to take many of them out. Palom's Flame magic and Porom's healing spells work well against the zombies that Cecil's sword can barely scratch. Or, to conserve MP for when you really need it, use the Flame Rod or more effective but more expensive Holy Arrows for damage, and the Healing Staff for recovery.

Again, the twins can't use bows in the DS remake. It also effectively nerfs the Healing Staff's item effect, leaving it weak enough that it takes about four uses to equal a single Cure spell. The Flame Rod, on the other hand, still works fairly well, especially if combined with Item Lore, though only against single enemies. Cecil's damage, meanwhile, is only halved against zombies, rather than essentially negated, which honestly feels like kind of a shame thematically.

The entrance is blocked by flames. That's no problem for the Nifty Exceptional Wizarding Twins, though.

Palom's freezing magic puts out the fire, and he starts gloating.

Porom hits him.

The EasyType simplifies おごりたかぶって (ogori takabutte, being proud and haughty) to うぬぼれて (unuborete, being conceited), and also adds an いつも (itsu mo) before おっしゃってる (osshatte ru), making it "The elder is always saying..."

On a side note, I always find it interesting, and somehow realistic, how much differently Porom treats Palom than she does anyone else.

Fade out and dramatic music change.

Change scene to a high-tech looking area. Rosa is tied up with a large metal blade suspended over her head (or a large metal ball in the ET and NA versions, a change that appears to be intended to make it more visible), while Cain stands guard and Golubaeser stands nearby.

The DS remake switches back to a curved blade, and also completely redoes the style of the location. It now looks like they're inside a generic unremarkable stone structure. I don't much like the change, though it does at least help the blade stand out visually.

Scarmiglione is a demon that appears in Dante's Inferno. The exact meaning of the name appears to be uncertain, but I've seen claims ranging from "trouble maker" to "ruffle-haired" to "poison-bearer".

The screen dims as a hooded figure appears before Golubaeser.

The EasyType replaces にぶろうというもの (niburou to iu mono, ought be but dull) with the more straightforward きかぬだろう (kikanu darou, ought not work well).

Scarmiglione disappears.

The EasyType replaces うやまうきもち (uyamau kimochi, feelings of esteem you have) with おもうきもち (omou kimochi, feelings you have).

４てんのう, or 四天王 (shitennou), a term with Buddhist roots, in practice roughly equates to "The Big Four" in English. While "Four Fiends" works reasonably well, it's too explicitly sinister, and while the part about elements turns out to be accurate enough, it's completely extraneous to the name.

The EasyType changes このあいだのような　しったい (kono aida no you na shittai, a foul-up like the recent one) to あんな　ぶざまな　すがた (anna buzama na sugata, such an unsightly showing).

Cain drops back and bows.

Scene change back to Cecil's group.

Many of the locations here are given names like "Mt.Ordeals-3rd station", originally しれんのやま　３ごうめ (shiren no yama, san gou me). While I'm not sure where "station" comes from (possibly reference locations in surveying?), it appears to be the standard translation of ごうめ (合目), which counts approximate tenths of the distance up a mountain. So, the 3rd station, for example, just means the location is about 30% of the way from the base to the summit.

A bit farther up the mountain, a familiar old man appears.

Palom was most likely about to say みはり (mihari, standing guard or keeping watch).

Porom hits Palom.

Tella glances at them, then turns to Cecil.

Palom walks over to Porom and says (presumably quietly).

It's significant that Meteor is not only legendary but also sealed. Things aren't normally sealed without good reason...

The EasyType replaces しょあくの　こんげん (shoaku no kongen, [he's] a root of all evils) with すべては　ヤツのせい (subete wa YATSU no sei, it's all his fault).

Tella should have a higher max HP than the twins, but other than that, he's rather less impressive now than when he first joined. The twins probably outclass him magically already, and they keep learning more spells and growing as they level, while Tella's spell list and attributes both stagnate. Still, he remains useful as a healer and secondary attack mage, and can use either your choice of rod to supplement damage or a Healing Staff to supplement recovery without using up his limited MP, plus he can tank a moderate amount of damage. Give him some better equipment from Mysidia for best results. He can't use arrows, though.

In the DS remake, Tella continues to make up for his limited spell list with comparatively high magic stats. He joined my party here with no equipment, which I suspect means he keeps whatever you had on him when he left earlier. Mysidia should have better armor for him either way.

Near the top of the mountain, a strange noise is heard.

There's a save point nearby. Always a good idea to use them, since they're usually there for a reason.

The noise is heard again soon after.

And again at a high bridge.

A cloaked figure appears.

Boss time. Although Scarmiglione comes with a pack of zombies, he is not actually Undead himself, and has no elemental strengths or weaknesses. The zombie horde is, as usual, immune to Dark and weak against Holy and Flame.

The DS remake makes Scarmiglione Undead, with the usual corresponding weaknesses and resistances.

This battle resembles the ones against squads of Baron soldiers in that the minions won't attack until ordered to or deprived of their leader, though here they cast Drain when triggered, with the following line from Scarmiglione, rather than simply attacking.

The best tactic seems to be having Palom hit them all with Fira, hopefully killing all the zombies, then Tella mop up with Fire if needed. Palom can cast Slow once or twice to give Scarmiglione fewer chances to act. Tella mostly stays on healing duty, though you may very well be able to manage with just a Healing Staff if you realize that Scarmiglione's main threat is counterattacking all attacks with Thunder. When left alone, he'll alternate between a normal attack and Thunder, and only use either when it's actually his turn. To minimize damage taken, attack as infrequently as possible. The easiest way to do this is to avoid attacking until Palom has used Bravado several times to boost his intelligence, then Pair Cast to wipe him out in one strike. Inflicting enough damage at once may skip his dying words, triggered when his HP drops below 1000 (it starts at 3500):

Whether the scripted death after his parting line does it or the damage kills him outright, the battle is won.

Though, as Porom outright muses in the menu in the DS remake, that seemed far too easy.

Don't cross that bridge just yet. Instead, save again, rest if needed, and reverse rows during the crossing, because as soon as Cecil sets foot on the far side...

The EasyType changes しして (shishite) to the more standard equivalent, しんで (shinde).

Scarmiglione appears behind the party, uglier than ever, and another boss battle, with more dramatic music, begins.

It's a back attack, so the front and back rows are reversed. If you already reversed them, though, that puts them back to normal. This time, Scarmiglione comes alone and Undead, with weaknesses against Holy, Flame, and Projectile damage, and absorbs Ice. His attack may also poison its target. Fortunately, he doesn't resist Dark, so Cecil's sword works just fine, and he doesn't have any special attacks except (maybe) the counter noted below.

Although it may be tempting to attack with Flame magic, be warned that he'll retaliate (original JP version only; he doesn't even react in the others):

As sinister as that sounds, it's effectively just casting Slow on the whole party at once. Still, better to avoid it.

Palom and Porom can deal almost absurd (if inconsistent) amounts of damage as archers. Holy Arrows inflict quadruple damage against Undead monsters to start with. Striking an elemental weakness here then doubles that again, for a final 8x multiplier. Even from the twins and their relative lack of skill with bows, that stings. Have them and Cecil attack while Tella takes care of healing, and the battle will be over in no time.

Also note that healing spells and items will damage him, but why bother? The twins can easily kill him by themselves with only a handful of arrows.

If you're wondering how these tactics work in practice, here are the results of two sample runs I did, starting with Cecil at level 19, Tella at 22, and the twins at 13. In the first battle, Cecil defended constantly, while Porom always healed with the Healing Rod until otherwise noted. Palom cast Fira, killing all but one zombie. Tella finished it off with the Flame Rod. Palom then used Bravado three or four times, and Porom initiated Pair Cast right after one of these, hitting for over 4000 damage and wiping Scarmiglione out instantly. Palom and Porom reached level 14, and Porom healed the group with Cure between battles to bring everyone up to full health. Porom switched to a Staff and gave the Healing Staff to Tella, while Palom took up a Crossbow and 30 Holy Arrows (since that's all I had on this save file). In the second battle, Cecil constantly defended again (just to see how well arrows really work here), and Tella took over healing with the Healing Staff. Porom started by casting Slow twice, then either used the Staff to cure poison as needed or cast Protect on the party to reduce damage taken (when the spell didn't fail). This left Palom attacking every round as the sole source of offense. A mere sixteen arrows later (with several misses), Scarmiglione went down, leaving the party only mininally damaged despite his best efforts to kill them. Another attempt with Cecil attacking and Porom not bothering with Protect had Palom spending just five arrows and left only Cecil at all hurt (a mere 26 damage). Note that in both trials, Tella spent no MP at all, and no one cast any healing spells during combat.

It's probably quicker to just demonstrate it, so here's a video doing exactly that.

That obviously isn't possible in the DS remake, with the twins unable to use bows, and Scarmiglione resists Cecil's Dark sword on top of that. I also saw him use the Slow-everyone attack as a counter to being struck once, so Cecil may be better off just defending. Porom and Tella can attack effectively by casting Cura, and Palom can use the powers of cockiness to cast boosted Fira spells. However, Scarmiglione can reportedly use a nasty mass status attack as a counter to Black Magic. I was lucky enough not to see it, though, and a boosted Fira hurts quite a bit.

He falls partway off the bridge...

...and into the ravine below. Regardless, a strange structure waits ahead.

And you might as well unequip Cecil before approaching, even though no one else can use the Dark Knight equipment anyway.

There's a glow, and the party is sucked into a small room with a large mirror for a back wall.

A sword descends, which Cecil takes hold of. Light surrounds him, and his appearance changes drastically. However, his reflection remains unchanged.

The EasyType avoids some relatively uncommon terms by changing かこと　けつべつする (kako to ketsubetsu suru, break with your past) to かこを　ふりきる (kako wo furikiru, shake free of your past) and こくふくしなければ (kokufuku shinakereba, if you do not surmount) to のりこえなければ (norikoenakereba, if you do not overcome).

Cecil's reflection steps out of the mirror.

The reflection gets in Cecil's face.

Cecil pushes his reflection back into the mirror, and they fight.

Cecil has become a level 1 Paladin with 600/600 HP, 10/10 MP, and the Cure spell, equipping only the Sword of Legend and Clothing, regardless of his previous equipment and level. Becoming a Paladin also changes most of his command list and his entire equipment set. If you bought a Shield of Light in Mysidia, you can put it on now, though it has no effect on this battle regardless.

If attacked, the Dark Knight uses Darkness (even in the English version, despite its removal of the command) as each of the following lines of text displays:

The best strategy is to do absolutely nothing. The Dark Knight will use Darkness repeatedly, dealing about 70~110 damage each time, regardless of defense (and again, this happens even in the English version). After the third time (not counting any during the above message)...

The DS remake seemingly does away with the counterattack, other than displaying the text. It also pays sufficient attention to detail to show the Dark Knight taking damage as he attacks with Darkness active, in amounts that dwarf what little damage Cecil can inflict by attacking.

...the Dark Knight disappears, and Cecil wins.

It is possible in theory to win by attacking, if you can manage to inflict 4,520 damage without dying in the process. I don't recommend trying it.

After more shininess and a fanfare, Cecil's reflection also turns into a Paladin.

The twins crowd around Cecil. Tella, on the other hand, looks distracted.

Considering the real reason the twins are here, Palom was probably about to say something along lines of, "You really meant that stuff you said to the elder? It wasn't all part of some elaborate ruse?"

Palom and Porom confer, quickly coming to a decision.

Tella rushes out.

He looks back and forth a few times, then hurries after Tella.

Porom leaves, and Cecil does too after taking one last look back. Incidentally, everyone is now at full HP and MP.

Give Cecil any armoring you have available. The gear from Mysidia is ideal, especially the shield for evasion, but nearly anything beats what little he's wearing. In any case, he can now use basic White Magic in addition to a much larger range of equipment, and his new Holy sword will devestate the local Undead monsters where the Dark swords barely scratched them, all while making a neat new sound unique to (non-arrow) holy weapons when hitting. He'll also gain levels rapidly for a while, hardly a surprise given that he started at level 1.

And after being upstaged by a pair of five-year-olds, Tella becomes impressive once again. His dubious Remember command is gone, and in its place, he can now cast nearly any spell in the game. Unfortunately, his attributes remain low overall, and his maximum MP never increases beyond 90, which can go by very quickly with the new high-end spells. It's still more than enough to give him a definite edge as long as he doesn't run out of MP at a bad time.

Out on the field, though, when you're not facing hordes of the living dead, you may find yourself missing some of Cecil's old powers. Without the Deathbringer, there's no more killing the Zuu birds in one hit, and they have a lot of hit points for this point in the game. Darkness worked wonders for mopping up large groups of weak enemies, too, but you'll have to rely on magic for that from now on, or take them out one by one. Even so, Cecil ends up being much stronger in the long run as a Paladin than he would have as a Dark Knight.

In the DS remake, Cecil needs a few levels under his belt before he can match his previous damage even against the Undead, despite now striking a weakness where before he was up against a resistance. Fortunately, those levels come quickly. Tella, meanwhile, keeps his Remember command, which in this version has a small chance of casting several rare spells that aren't learnable. His solid magic stats in this verson also make his new spell list all the more formidable.

The party returns to Mysidia, optionally taking a chocobo from the forest south of the mountain.