Coal used for weapon infusion.
The white magic flame produced by this coal was given to the Undead Legion long ago by one of the Crystal Sage twins.
Give to the blacksmith in the shrine to allow the use of gems for crystal, blessed, and dark infusion.
Found in Farron Keep. Starting from the first bonfire, keep to the left-hand side until you reach the small structure guarded by a Darkwraith. Sage's Coal can be found within structure.