Blacksmith McDuff is the only blacksmith in the game that can infuse equipment. Infusing costs 2000 souls and the corresponding Infusion Stone, removing an Infusion costs 1000 souls and a Palestone.
He must be given the Dull Ember before his services are made available.
In order to get the "Smith for Life" Trophy/Achievement, one must spend a certain amount of souls (14,000) and/or use every type of infusion (varies depending on version, PC SOTFS requires infusions), and then talk to him.
You can enter McDuff's room in the following ways:
The Bastille Key will allow you to simply open the door.
Push the explosive barrel down the stairs to destroy the weakened wall to the left of the door, revealing a bonfire and the rear entrance to McDuff's smithy. If the barrel does not explode on its own, you can simply nudge it into place and detonate it yourself with a Firebomb. May be able to set off barrel with any weapon that does Fire damage. A sufficiently large enough weapon, such as a Great Hammer or Ultra Greatsword should also work.
Once the torch in his room is lighted he will move near the torch to work on anvil leaving the chest he was covering before open for looting.