The Phantom Blade Zero Heart of the Fist side quest is a very simple one, tasking you with finding the grave of the Iron Fist Master and leaving some incense there for the Iron Fiend. Unfortunately, the only hint that the game gives you is that the grave is in the Mirrorwood. Thankfully, it's not very far at all and you will have walked past the grave at least once or twice while playing through Phantom Blade, so this is where to find it.

The Iron Fist Master's grave is very close to the Burial Grounds Bell on the eastern side of the Crimson Forest. Fast travel to this location, then head over to the large grave in the middle of this icy clearing.

Interact with this grave and you'll see a cutscene of Soul burning some incense to honor the fallen master. You'll then immediately be rewarded with the Fist Fragment: Shell item which automatically combines with the Fist Fragment: Core you got after fighting the Iron Fiend the first time. They form the Worldbreaker Fist , which is the necessary item for Soul Tempering the Iron Gauntlets Phantom Edge. And that's the end of Heart of the Fist! Now you should carry on exploring the frozen parts of the Crimson Forest. I recommend going back to the Sha-Chi Tower and sparring with the Jade Dancer and Iron Fiend for the 'Use It Against Them' quest if you haven’t already, as this is how you get the Soul Tempering item for the Fiend Saws dual swords.

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