In Octopath Traveler, you can experience the secret ending and unlock the Dungeon but for that you have to be more vigilant. In order to unlock the Dungeon you have to do the following:
Complete all the stories: Here you have to play all four chapters of each character to make it to the end.
Lyblac's Quest Series: In Bolderfall you will find Ravus' Estate, where you can find the Alpha. Challenge him to a fight (you can use either Olberic or H'aanit), then go to Saintsbridge. Namely Alpha sneaked back there and devoted himself to his poetry. To learn his poem, you must address him with Alfyn or Cyrus and then take the said poem to Flora. She is waiting in the cathedral at the bridge.
Kit’s Quest Series: Kit meets you early in the game, shortly after you leave your first city. He will ask you for a cure. Give it to him and then meet him again later in Noblecourt. Close by there is an actor whom you, Ophilia or Primrose, have to do to encourage you to follow you to bring him to Kit. This concludes an important part of the quest. Next up is Grandport, where you'll find a dealer in front of the tavern, from which you'll get lazuli as Therion or Tressa Lapis, which you bring back to Kit, who is now in the beach of the moonstruck.
Once you have completed the stories and quests, you will have to find the western S'warki Trail and Impresario. Then you'll encounter mini-boss, talk to the impresario again and unlock the last and secret dungeon, the ruins of Hornburg.
Ruins of Hornburg
Here, you’ll encounter some bosses. Since you will not be able to save or leave the area to get some helpful items, you should be well prepared to fight.
The battles can be started as soon as you visit the blue flames and interact with them. Then you will be taken to the boss without further ado. With each win, the flame goes out and you can instead open a text in which you will learn even more background stories that are bound together piece by piece, the eight loosely linked characters and their respective affiliation in the story is explained.
If you have been able to put out all the blue flames there and defeat the bosses, a door will appear on the top landing, through which you can reach the final end boss of the game. For this you need at least two groups, so make sure that the strengths and weaknesses are equally distributed. The fight is divided into two parts, so you use your first group for the first part and the remaining four for the second part.
Don’t assume that you can do the boss immediately in the first round. After all, you don't have to do all the bosses again once you fail. After the eighth Shadowboss you can leave the ruins, save and collect items before you finish the game.