Octopath Traveler 0 Endings: How to access each ending in the game
This page will have spoilers for Octopath Traveler 0.
As you near the finale of a lengthy multi-arc story in Octopath Traveler 0, you'll soon need to make one last decision before you hit the final credits of the game. However, at first glance, it may not be obvious where the decision is or how to make it. This page will outline how to get the different endings in the game. I'll try to be as light on spoilers as possible, but not too light as to be vague.
There are two endings in Octopath Traveler 0. Which ending you get depends on your actions in 'the lone house in the snow', accessed just before the final boss of the game. This place doesn't have a given location name, in-game, so I will simply refer to it as 'the lone house in the snow'. You cannot miss this place before the finale.
It is also very important to mention this: when you save a clear file for Octopath Traveler 0, a new pedestal appears in the final dungeon, just before the warp altars (at the 'Garden of Sin'). This new blue-colored pedestal (it looks like a standard Save pedestal, except blue) allows you to rewind time just before the decision point. This effectively means you can re-decide your ending path. You can clear either of the two endings as many times as you'd like, and neither ending will prevent you from obtaining the other one.
Ending 1: Champion of the Continent
I'll consider this the 'Standard Ending', as this is the path you will default to if you do not trip a few optional flags. In order to get this ending, when you reach the lone house in the snow at the end of the Heavens dungeon, simply leave the house. The NPCs here will interject several times, but ignore them and just leave. If you have not powered up your ring (see the section for the next ending for details), then this is the only choice you can make.
Once you leave the lone house and return to the Heavens, you'll battle one of the game's final bosses - Bestower of All.
Completing this ending will get you the Champion of the Continent trophy/achievement.
Ending 2: Traveler from Zero
This ending feels more like a "True Ending", given the additional requirements necessary to achieve it. Accessing this ending will also give you your final Traveler in the game, unlocking the The Summoned trophy/achievement for allying with all travelers (assuming you got the rest).
The key to accessing this ending is that, before going to the lone house in the snow, you need to return to the Flamebearer's Shrine: Sanctum and interact with the big blue flame there. The game does not place an objective marker here, but if you interact with the flame, you'll fight an optional boss encounter. Once you win, the protagonist's ring will be powered up.
Assuming you've seen this event at the Flamebearer's Shrine, return to the 'lone house in the snow' - using the 'rewind time' blue pedestal in the Garden of Sin if you must. Now in the house, you'll get a new Path Action choice to 'Illuminate' the three NPCs in the house. Do this, and you'll be on the path to this ending. You'll fight a different boss, and you'll obtain the last Traveler.
Note: While the main character's ring will "break" from these events, preventing them from using ultimate abilities, you can simply return to the Flamebearer's Shrine to restore it.
Completing this ending will give you the Traveler from Zero trophy/achievement.
(Editor's Note: It's possible Ending 2 will also give you the 'Champion of the Continent' achievement, I can't be certain.)
Post-Game
Once you complete both endings to Octopath Traveler 0, you'll be able to fight a new ultimate boss in the Cove of Clouds, accessed from the Skies of Vanity area of the Heavens dungeon. This will allow you to fight Amatsukami no Orochi as your final challenge, which allows you to get the Dragonslayer trophy/achievement. You'll also be able to pick up a Floating Stone, which is likely the last Legendary Treasure that you'll need for your collection.
Beating Amatsukami no Orochi also rewards you with a Heavenly Jewel, which once placed in the Strange House, let's you finish the last few entries of your Music Score collection. You'll need to have found your way to Agapea to do this.