Total Conversion: Ys III vs. Ys: The Oath in Felghana
In Ys III, Adol heads back to town to talk to the Mayor. In OiF, Berhardt charges out of his hermit cabin and offers to help defend the town. In the meantime, Chester, all beaten up, gives Adol the sword he needs for the final battle.
Adol sets off to Genos Island on boat. It's simply called Galbalan (or Demanicus) Island in Ys III. In OiF, the ship is manned by Dogi, who's still weak from his injury and thus can't really do a whole lot.
The shore of Genos Island doesn't have anything really interesting. In Ys III, the entrance to the cave had rocks in the shape of teeth, which opened up when presented with the correct item. There is nothing of the sort in OiF....
...but OiF does have another brand new boss battle where you fight against Dularn, who reveals himself to be...a herself, technically, as one of the nuns in disguise.
Genos Island is totally different in both versions. In Ys III, it's entirely dark, except for a spotlight activated by an item. This is one of the few areas that's actually kind of a maze, because it's possible to circle around infinitely if you're not careful, although it is actually quite short when you know what you're doing. The biggest pain is identifying the parts of the floor that are actually elevators, because they're not immediately apparent. Well, OiF disregards all of that, it's just a really long dungeon with lots of tough enemies, more obstacles (the spiked spinning logs can prove to be a pain) and more platforming. It does look really cool though.
The order of some things here are slightly different. In Ys III, you fight Garland first (below) and then ascend a tower (with a cool spiral effect), which brings you to the encounter with Demanicus. In OiF, you instead descend a spiral staircase into the ground, fight Garland, and then go to the Demanicus encounter. Here's the staircase, in both situations.
This is one aspect I really missed about OiF. This next-to-final-battle is fought against that same red background with the pillars as featured in the prologue. Well, it's never clear in OiF that this takes place in the same room, so that effect is lost.
The whole bit with Elena and Chester is a bit different. In Ys III, Chester attempts to save Elena himself, but Adol instead goes after Galbalan. In OiF, Elena is simply tied up, as her blood is powering the resurrection. Chester does show up, eventually, but not until Adol goes to meet Galbalan.
In Ys III, Adol rides a beam of light into the sky to meet Galbalan. In OiF he's lowered into the ground on this pillar.
And here's the final fight. In Ys III, this is a ridiculously hard battle, because visually it's just too busy. In additional to the striped scrolling effect in the background, there's all of the lasers and bits of rock flying around (which don't actually hurt you, at least.) Plus, Galbalan is airborne, and hitting him is ridiculously hard. The whole battle was redone in OiF, with the final part involving playing ping pong with energy balls.
This just look too cool. Some of the later bosses have Final Fantasy VII-style limit break attacks where they trap you and do some major damage, all with lots of cool visual effects. This is Galbalan's.
Elena, Chester and Adol discuss what to do after the fight.
In the end, Chester still sacrifices himself to destroy the island.
And so it explodes.
Adol in the ending of Ys III is a real jerk. He decides to leave early in the morning so he doesn't have to say good-bye to everyone. (This all becomes skeezier if you jump to the conclusion that Adol and Elena knocked boots the night before.) In OiF, it's totally the opposite, as Adol is first seen trying to talk to Elena, who refuses to come out of her house due to all of the trauma. You're given the opportunity to talk to all of the townspeople again.
In Ys III, Adol says goodbye to the guard, while accompanied by Dogi. In OiF, Dogi stays behind to help rebuild the village, but pledges to catch up later. In both versions he urges Elena to chase after Adol.
At this point in Ys III, there's no more text, just some random shots with Adol, Dogi and Elena, which assumes that Elena caught up to Adol to say her last goodbyes. In OiF, she's explicitly shown to speak to him before he takes off, where she thanks him greatly.
After the end credits roll in OiF, you see the final sword amidst the wreckage of Genos Island.
Thanks to MP83 for his extensive Ys knowledge and proofreading skills, which helped contribute to this article.