Delrith

Delrith is the demon summoned to destroy Varrock originating from the infernal Pandemonium Realm. In the past, he was banished by an ancestor of Grufeld Bach and when a gang of Zamorak Monks led by Denath summons him again in 169, an adventurer dispatches him during Demon Slayer.

He has a combat level of 15 (60 in Dominion Tower) and appears in the crypts below the Zamorakian church in the Demon Slayer quest.

In battle Delrith uses Melee and Magic. His most powerful attack, is a power spell he uses to try to incenerate his opponents. If not protected against using Bach's barrier, it will reduce your lifepoints to 1 regardless of your current life point total. Even with protection, it will still cause nearly 20% damage to your current lifepoints total.

The demon can only be attacked with Silverlight, a weapon enchanted by Guthix to maintain the balance of Gielinor, and no other weapon or the message "I'd better wield Silverlight first" appears.

Delrith is fight-able as a class H boss monster in the Dominion Tower minigame if the player has completed Demon Slayer. A copy of Silverlight is provided within the arena.

Delrith is known for boasting of his ability to destroy Varrock, despite his low combat level and life points. In a Postbag from the Hedge letter that brought this point up, Delrith replied, "To cause widespread fear, you don't need strength; you just need the impression of it. Look at me - I am terrifying to behold! No fool could be blamed for fleeing from me. Few stick around to face me in battle once they have beheld my horns and my wild-eyed stare."

Nonetheless, Delrith at full power is terrible to behold, and dreaded by all other forces as a mighty bane of light. Before the Demon Slayer update, Gypsy Aris described him as "a powerful demon", and believes him capable of destroying mankind's most powerful and prosperous city, and also fears him, even when he's upon another plane, sensing her peering at him through her crystal ball.

History
It is implied by the Demon Butler that Delrith originated from the Infernal Dimensions as a Demon Lord whom he served under, and in Demon Slayer, it is indicated that the dimension Delrith is entering Gielinor through is actually "Zamorak's realm."

Many years ago, during the late Fourth Age and early Fifth Age, the druids of Taverley had many stone circles built across Gielinor. At these sacred altars the druids would worship Guthix, the god that shaped, but not created the planet. One such circle was found just outside of Varrock, the massive capital of Misthalin. For some years it stood as a beacon to the followers of Guthix.

The circle was destined to fall, however. 150 years ago, in the year 19 of the Fifth Age, the stone circle was captured by an order of dark wizards, who, for unknown reasons, sought to destroy Varrock in Zamorak's name. It was here that they summoned Delrith although he was swiftly defeated by an ancestor of Grüfeld Bach.

Demon Slayer
During Demon Slayer, a cult of Zamorakian Monks and a man named Denath prepare to summon Delrith once more in the Church of Zamorak in the southeast of Varrock by stealing a tome from the monastery near Edgeville. Bach, who had been following Denath for years and occasionally thwarts his plans, followed Denath to Varrock where he recruits an adventurer to aid him. Once the adventurer accepts the offer to help, they begin Demon Slayer.

To defeat Delrith, the adventurer must acquire Silverlight, while Bach tracks down Denath. After dispatching Denath's followers, Bach and the adventurer reach Denath who manages to Summon Delrith. Between the adventurer's attacks wielding Silverlight and Bach's massive protection prayer, they manage to defeat Delrith once and for all. Once Delrith is defeated, the cave under the Zamorakian church collapses, but Bach and the adventurer manage to escape.

Trivia

 * During the job interview in the Servants' Guild, the Demon Butler states that for several centuries he was Delrith's private valet.
 * In the 3rd Postbag from The Hedge, one player asked about Delrith being "naked" and he responded that he is embarrassed about being naked and immediately sends out one of his wizards out to get new clothes for him. And that when he sees his reflection in the blade of Silverlight his combat level drops down to 27 because of his embarrassment. This, of course, is just a joke and shouldn't be taken too seriously.
 * In RuneScape Classic Delrith had far more than 70 life points. Some players engaged him with a rune weapon, and then switched to Silverlight when Delrith had low health.
 * Before the graphical update of Varrock, Delrith could be seen walking in his usual spot, even after killing him.
 * Delrith was the first strongest quest monster fought in RuneScape. Later, when the Vampire Slayer quest was released, the Vampire, Count Draynor, succeeded him as the strongest quest monster.
 * Delrith is currently bugged in Dominion Tower where his body parts appear stretched out and disfigured when not performing an animation.
 * Interestingly, he used to have have less health than a level 3 player just starting to play, and even Imps had more health than him, This is due to the fact that he has been freshly summoned and, as a result, still weak from his movement between dimensions.