Nailsmith

"Follow a twisting path from the outskirts of the drowning city. Cross a broken bridge and you'll hear it close, the ring of metal and the roar of flame. A craftsman obsessed.  

''Within his forge the Nailsmith is searching, on an endless quest to create a flawless nail. ''

''Past failures pile outside his door, exposed to the rains and left to rust. He spares no thought for imperfect things. Nor does he pause to take notice of the countless beings that take their pick of his refuse. ''

''But his neglect has impact. His discarded nails have spread wide through Hallownest, clutched tight in the hands of creatures possessed. ''

''Only in the Hollow Knight's broken nail does he at last see true potential. Distant and difficult to achieve, but it is in there, waiting - a blade to be honed beyond all others, to be forged into a form so brilliant it transcends its metal and becomes myth." ''

Found in the City of Tears at the far left of the map from the bottom. The Nailsmith is indicated by a nail sign pointing to his house near the elevator at the entrance of the City. He'll provide the first nail refinement for Geo only, after which he will require Pale Ore and Geo to upgrade further. See Nail for prices.

After forging the Pure Nail, the Nailsmith will step outside and asks you to cut him down. Obliging him sends him plummeting down to the waters below, his corpse is not found.

This will award you the 'Purity' achievement. If you do not grant his request however, he can be found with Nailmaster Sheo and you will be granted the 'Happy Couple' achievement after talking to him.

Trivia
The Nailsmith's quest to forge the perfect nail was a Kickstarter stretch goal at $44,000: One of the stretch goals at $44,000 was the addition of four new quests to the game: