For Reinforcement of the Demon Weapons and Shields. Almost all of these items are created from the Soul of a slain Demon and a standard +10 Weapon by the Giant Blacksmith.
All items can be Reinforced to +5 and will take 10 Demon Titanite to get there. Most items will also cost 5,000 Souls to upgrade at each step, with the odd item costing either 2,000 or 10,000.
In most cases, each upgrade will improve the items Damage attributes.
The Giant Blacksmith, found in Anor Londo. Sells a number of high-end items and can do some real nice work on your weapons too.