There are three different Phantom Blade Zero endings; one bad, one slightly less bad, and one True Ending which lets you face the final boss, granting the "Final Battle" ending and "With Heart and Blood" achievement. But to get this perfect ending, besides following the main quest, you have to complete a variety of side activities.

The cut-off for completion seems quite generous—though Inanis implies there's a point of no return just as you enter the Mirage Estate, I completed some of these steps later and I was fine. Here's what you need to do

You first get this quest when you discover one of Firefly's Memory Flowers in the Village of Lu, but the main steps are:

There are five Long Pillar challenges battling the Gold-Masked One that you need to complete to get the True Ending, plus these are good as the later ones give you Soul Tempering materials for some of the game's best weapons:

Though I haven't seen this requirement mentioned elsewhere, I'm pretty certain you have to complete the Quickhand's Request quests for the True Ending, which involve finding treasures throughout each region and delivering them to the thief.

This is all covered in the linked guide with videos showing the locations, plus this quest gets you two otherwise inaccessible Phantom Edges, Flying Dagger and Goldblaze, so is worth completing anyway.

If you've done everything above, Inanis should tell you right before you emerge onto the peak of Mount Soulrend (after the Mirage Estate) that there's nothing more she can do to prepare you.

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