In Game Description
Ember required for weapon ascension.
Huge ember of highest quality.
Handled by the blacksmiths of Astora.
Ascension for +10 standard weapon.
Allows reinforcement to +15,
on par with the greatest weapons of legend.
Found in New Londo Ruins
After lowering the water level, proceed through the newly drained area until you reach the building with the first Mass of Souls. Take the staircase near a Darkwraith and follow the path and climb the stairs to a chest containing the ember.