Darklight

Darklight is a magic sword created in the Shadow of the Storm quest. In the quest Silverlight becomes Darklight by 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, the player's maximum hit is increased by a fair amount, approximately to the same strength as a dragon longsword. Darklight's accuracy, however, remains the same which means you can still miss often.

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

Tormented Demons
Darklight is also used to remove the fire shield around Tormented Demons, weakening them considerably.

Retrieval
If you lose Darklight, you can retrieve it at Varrock Castle. Talk to Sir Prysin and pay him 1,000 coins. You can obtain multiple Darklights by using the Drop trick. If you use the drop trick in f2p, Sir Prysin will say he cannot trade you because it is a members only item.

There is a glitch where you can have both Silverlight and Darklight, you need to get Darklight while having a silverlight in your POH

Trivia

 * Darklight is an Oxymoron.