To get Sword of Darkness you first need to obtain the Stone Sheated Sword
There are three locations you can get it from
1. Ruins of Unte - Behind the Furnace Golem if you wake him by throwing a Hefty Furnace pot at him
2. Ancient Ruins of Rauh - Invisible Bridge
3. Fog Rift Catacombs - The second spike trap where the sorcerors are targeting you. Once you have dealt with them, hop on to the top of the spike trap and go forward. On your left, you'll see a room with a sword in it. The catacombs themselves can be found on the narrow path left of Castle Front Site of Grace.
Note you only need to get the sword by any three of the methods
Now once you have the sword you need to visit any of the other two altars and make it Sword of Darkness.
In sumamry get the sword from any of the three Altars and imbue it with darkness from any of the remaining two