Darklight

Darklight is a magic sword created from the sword Silverlight during the Shadow of the Storm quest. In that quest, Silverlight is transformed into Darklight after being dyed with black mushroom ink and then being used to destroy the demon Agrith-Naar, which coats it in ichor (demonic blood). Both Silverlight and Darklight can be mounted on the wall of a Quest Hall in a player's home. This weapon cannot be made using the Smithing skill.

When using Darklight to fight Lesser, Greater, Black or Abyssal demons or Bloodvelds, the player's maximum hit is increased to approximately the same level as a dragon longsword, however it’s accuracy remains the same as Silverlight’s. Darklight can also be used to remove the fire shield from around Tormented demons, which significantly reduces their defence.

Darklight has a special attack, Weaken, which lowers the opponent's Strength, Attack and Defence levels by 5% (10% for demons).

Retrieval
If a player loses Darklight, it can be retrieved by talking to Sir Prysin in Varrock Castle and paying him 1,000 coins. Multiple Darklights can be obtained by using the Drop trick, but if a player is on a free-to-play world as a member, Sir Prysin will say he cannot trade with the player because Darklight is a members only item.

The only way to retrieve Silverlight after completing Shadow of the Storm is to become a free-to-play player, then visit Varrock Castle. Only then will Sir Prysin agree to sell Silverlight.

Trivia

 * Oddly enough, the Varrock Museum display plaque calls Silverlight, and therefore Darklight as well, a sword made of steel. This cannot be since both Darklight and Silverlight may be used to slay Vampyres, who can only be damaged by weapons made of silver. The discrepancy can be explained by the ‘magical treatment’ alluded to by the Varrock museum's display and by the God Letters, where Guthix claims responsibility for the creation of Silverlight to restore balance following the appearance of Delrith.