Quick answer: After the ending, talk to Inanis in the Realm of Mirage and pick New Cycle to start New Game Plus. To keep playing, pick Leave instead. The cursor starts on New Cycle, so press Down before you confirm.
Endgame story spoilers follow.
Where to start New Game Plus
What Inanis says before the choice
According to Soul, the Lady was not responsible for bringing the Blight into the Waking Realm; countless lives were saved.
Inanis tells him the Flesh Sculptor could replace his damaged heart with the mechanical heart.
She says he could live for decades longer, with Sha-Chi and martial ability unmatched in this world, although some might regard this as a form of Transcension.
In a brief scene, Soul considers the offer. He recalls the temptation once faced by the heroes of the Mandate and wonders whether he now has to relinquish the heart he found.
In our run, talking to her a second time skipped the talk and went straight to her question and the same menu.
New Cycle or Leave
You can choose only New Cycle or Leave; neither comes with a description.
New Cycle is the game’s term for New Game Plus. Your cycle count appears on the end screen’s Cycle line.
The cursor starts on New Cycle. Press Down to move it to Leave before you confirm, because confirming straight away picks New Cycle. In our run, the cursor sat on New Cycle every time the menu opened.
Leave closes the menu and returns control to you in the Realm of Mirage. You can talk to Inanis again later.
How to keep playing after the ending
There is no bell in the Realm of Mirage; to get out, return to the islet that holds the portal and use the blue swirl marked with an E prompt.
In our run, pressing E at the swirl took Soul to the Puppet Cell in the Mountain of Imprisonment, next to a pagoda gate with an orange glowing portal, not back to the site of the last fight.
The Puppet Cell bell is a short walk from there. Its menu lists Traverse, Upgrade Equipment, Store, Train Abilities, Save and Leave.
Traverse remains available after the ending. It brings up the map for the Mountain of Imprisonment, while Tab opens the world map showing every region. In our run, Traverse took Soul to bells in Weeping Creek Valley and Pangzhen after the ending.
Quests after the ending
In our run, the Story tab showed A SOUL’S WORTH finished, beneath its line “The Loong’s roar beneath the abyss”.
In our run, Against the Flow was the only story quest still open. Its page read “Find the Great Waterfall”, with a marker in Weeping Creek Valley. The story walkthrough covers the details.
In our run, tales that were open before the ending stayed open on the Tales tab, including Triple Threat, Hero’s Lament and The Sword and the Way. The full list of tales has the rest.
Difficulty and appearances after the ending
Finishing the game unlocks the 66 Days mode. Its description in Select Difficulty reads “Same difficulty as ‘Hellwalker,’ but deaths are limited to 66 times. Unlocked after completing the game on any other difficulty.” The difficulty modes guide covers every option.
The Final Battle ending also adds three appearances, The Hunt, Kindred and Blizzard. You change them from the Character menu, and the outfits guide lists them all.
