This looks like a carpenter's hammer of great craftsmanship, and if studied closely, faint runes can be seen embossed upon it. The hammer will radiate magic if tested.
Anyone with carpentry or building skills (or, at the GM's discretion, elves or other creatures known for their wood craft), can use this hammer to build wooden objects and structures very quickly. Using it to build a large house will result in the house being completed in only a day or two. A chest of drawers, or an armoire might only take an hour. In fact, anything built with this hammer almost seems to build itself, and will, at first, be quite sturdy.
Every day after the object or dwelling is completed, however, there's a cumulative 01% chance that it will fall apart. Once per day, the GM or the owner of the built object must roll 1d100; any roll equal to or less than the current cumulative number means the crafted object has collapsed. Nothing built with this hammer, therefore, will still be standing in just a few months. Up until the moment of collapse, however, the built object will seem to be just as solid and dependable as ever.
Optional: Should this hammer ever be used in combat, it is considered to be +1 to Hit and Damage, and does 1-3 HP of damage.
Optional: Should this hammer actually do any damage in combat, the damage will just fade away in minutes; effectively, the creature struck will heal that damage back in 1-3 minutes. Should this bring them back up over zero HP, they are alive once more!