Magic Weapon Achievement in Dark Souls
Acquire best weapon through magic reinforcement.
How to unlock Magic Weapon
Large Magic Ember: Found in the room where you fight Seath for the first time. The ember will be in a chest that will only appear AFTER Seath is defeated.
Rickhert of Vinheim: Located in New Londo Ruins. Right before you go into New Londo, go allthe way to the right and down the staircase to the left to see him locked up in a cage. He is safe and he will remain there forever (unless you attack him, in which case he will run back). Give the Large Magic Ember to him.
In order to get a +10 magic weapon, you need 10 green titanite shards (purchased from giant blacksmith in Anor Londo), 7 blue titanite chunks (farmed from blue crystal golems in Darkroot Basin/Crystal Caves/ Duke's Archives gardens), and a blue titanite slab (farmed from moonlight butterflies in Crystal Caves or found in the Crystal Caves. Drop down into the invisible path and turn around into the cave with 3 crystal lizards and follow the invisible path out of there to find the corpse with the slab).