Lightning is an upgrade path offered by the Giant Blacksmith in Anor Londo through ascension and does not require any ember. To create a Lightning weapon, the player must have a standard weapon +10, Titanite Chunks up to +4, and a Titanite Slab to upgrade to +5, as well as the requisite souls.
The Lightning upgrade path adds Lightning Damage to a weapon, while removing all stat scaling bonuses. This makes Lightning a good choice for those who lack the stats necessary to bring out the full potential of a weapon. Lightning weapons are thus very popular for lower level (and even some higher level) PvP.
Even at higher levels, Lightning is still useful; the damage is only slightly inferior compared to fully upgraded Standard weapons with maximum scaling, and most armor and shields have poor Lightning Defense.
The Lightning path's largest weakness is its inability to be augmented. With good scaling, Standard weapons with even relatively minor enchantments such as the Gold Pine Resin deal more damage than Lightning weapons.
In the Artorias of the Abyss DLC, the lightning upgrade path was modified, dealing less damage.