Large Magic Ember

In Game Description

Ember required for weapon ascension.
large magic ember, a form of sorcery.
handled only by the Vinheim blacksmiths.

Ascends +5 magic weapons.
Allows reinforcement to +10,
the most powerful level for magic weapons.

Availability

Found in The Duke's Archives, after defeating Seath the Scaleless

It is inside a chest in the room where Seath the Scaleless is encountered the first time

Large Magic Ember location

Use

Given to Rickert of Vinheim in New Londo Ruins to enable further ascension of Magic weapons