Dark Lord

The Dark Lord was a mysterious entity whom the Iorwerth Clan tried to summon to Gielinor and who is encountered and fought at the climax of Plague's End. It was formerly part of Seren's psyche, the part of her mind fascinated with death, and the death of her elven followers in particular.

Creation and Entrapment
The Dark Lord was created when Seren poured her negative influences into her proxy Haluned after having heard the ultimatum of Guthix at the end of the God Wars, so that Seren could safely shatter herself without any remaining negative energy threatening her elves or Prifddinas. After being split off, Seren ordered Haluned away, where it made its way underneath the Galarpos Mountains, attracted to the death energy of the Death Altar, although it was unable to access its power. The Temple of Light was built around it by the Hefin Clan in an attempt to seal the Dark Lord there. From within the temple, it began whispering to people in the Underground Pass, driving them mad in an attempt to see more interesting deaths.

Recruiting the Iorwerths
Eventually, Lord Iorwerth came into the service of the Dark Lord, and worked with King Lathas Ardignas in an attempt to free it from the Temple of Light. Lathas told an adventurer that his brother King Tyras, was forced to drink from the Chalice of Eternity by the Dark Lord, corrupting him in the process, but this was a lie. Both King Lathas and General Hining assumed the Dark Lord to be Zamorak, but they were proven wrong when Zamorak returned to Gielinor in the Sixth Age and the Iorwerths continued to attempt to summon the Dark Lord.

Iorwerth established the Prifddinas Death Guard and had it set up base in West Ardougne under the premise that a plague had infected the city, and began kidnapping citizens to use as slaves to dig into the Temple of Light to free the Dark Lord and use the power of the Death Altar to empower it enough to restore Prifddinas without the aid of the other clans, leaving the Iorwerth clan as the sole ruler of the city.

Defeat
By the time the slaves reached the temple, its safeguards had broken, although an adventurer was quickly able to restore them, preventing the Iorwerths from getting in and using the Death Altar to give the Dark Lord enough death energy to restore Prifddinas. Iorwerth's next plan involved slaughtering the entire population of West Ardougne to create a roughly equivalent amount of death energy, but this was too foiled by the same adventurer.

Ultimately, the adventurer and the other 7 Elf Elders, with Iestin Edern standing in for Lord Iorwerth, broke all the seals to the Grand Library, and began to restore Prifddinas. A combination of the deaths during an ongoing battle between the Tyras guards and Death Guard, the breaking of the seals to the library, and the death of King Lathas (either by the Dark Lord or an adventurer) gave the Dark Lord enough to manifest within the Library and attempt to stop them restoring Prifddinas, both attempting to corrupt Prifddinas and preventing light from reaching the Prifddinas seed. Once manifested, he also summoned Lord Iorwerth and killed him to become even more powerful, with the Dark Lord referring to the deaths of a king and an elven lord to be "delicious". By attacking the Dark Lord, the adventurer was able to weaken him enough for light to pass through him, which both weakened him and allowed the rest of the seals on Prifddinas to be broken. Ultimately, his manifestation is defeated as the final seal is broken. His fate after this was unknown, that he may have been either completely dead or once again sealed within the Temple of Light.

The Light Within
During the attempt to restore Seren, the adventurer is reminded the Dark Lord, Haluned, was a part of Seren as well. They then visit Morvran to help enter the Grand Library to find what remains of the Dark Lord, as a single remnant of a dark crystal. The adventurer has a choice to decide if it's best to complete Seren with the dark crystal, or without, kicking it into the dark abyss below. If restored within Seren and revived on Freneskae, she'll accept that she should never have hid from her inferior past, never to have removed it, rather take heed on to protect her elves by controlling her dark side.

Trivia

 * Due to being a fraction of Seren, it ranks as a tier 7 divine entity on the tiers of godhood.
 * "The Twilight Twain", a music track exclusive to the CD given to those who attended Runefest 2014, includes lyrics that somewhat describe the Dark Lord's appearance. For example: "Weeds become sinew, thorns a spine. Crest displaced to a crown. Legs dark as crystal, arms thick as vine. Shades from the Well as its gown."