The Dagon'Hai (quest)

Starting out
Talk to Katsume in the great hall of the Legeslatum HQ.

K: [Player name]! Good to see you again!

P: You too Katsume. Has there been anything going on since we last met?

K: Actually [Player name], I was about to send someone to find you so that you could help with the latest threat to Gielinor

P: What's that then?

K: The Dagon'Hai

P: That's impossible! Their leader is in prison, I caught him myself!

K: Try telling that to Sil'as Dahcsnu

P: What has he got to do with it?

K: He has taken over leadership of the Dagon'Hai and begun to work on something more than taking Varrock

P: Really? What?

K: Sil'as has set his sights on all of Gielinor, he plans to become the complete and utter ruler of this world!

P: How does he plan to do that?

K: He plans to revive the second dragon ever created, the original King Black Dragon!

P: The original?

K: There has only ever been one Queen Black Dragon however over the years there have been many King Black Dragons, the first one was called Ladon

P: How do you know Sil'as' plan?

K: Friends in high places (he beckons to a man)

K: This is Dakh'thoulan Aegis, chief advisor of the lord of the Dagon'Hai

P: I always knew you weren't a true Zamarokian!

D: You dishonour me [Player name], I am a Zamarokian and proud. Just because I am Zamarokian does not mean I cannot have friends who aren't.

P: Sorry

K: Aegis has provided us with invaluable information on Dahcsnu's plan, tell him

D: In order to revive Ladon, Dahcsnu needs the head of a King Black Dragon as an offering to Ladon

P: Why are you willing to betray your leader?

D: I believe that if Sil'as can be defeated, as the highest ranking member of the Dagon'Hai, I would take his place

P: In any case Sil'as is out of luck, I have the heads of the current King Black Dragon mounted on the wall at home

D: Go and check if they are still there, quickly!

The Theft
Go to your POH to wherever your KBD heads are mounted and you will see a Dagon'Hai prising it from the wall while four others watch

P: Hey!

The one with the head teleports away and the others turn around to attack you, they are all level 75 and attack using melee and fire spells. Once they are defeated go back to Katsume.

Preparing for war
P: The Dagon'Hai took the heads

K: Then Dahcsnu will have already started the ceremony. Me, Aegis and the rest of the Legeslatum will meet you outside the entrance to the chaos tunnels near Varrock. Go prepared, this will be a long fight.

The Entrance
Once you are geared up head over to the Saradomin statue near Varrock and speak to Katsume

K: Are you ready?

P: As I'll ever be

K: Then let's go

Go down into the tunnel and go to the room which has the chaos altar portal in it. At the far side of the room there is a hole in the wall flanked by two level 95 Dagon'Hai guards who will attack you on sight. Kill them and go through the hole, a cutscene will begin.

Ladon
Sil'as Dahcsnu is standing on a floor of dirt with a circle of Dagon'Hai

Sil'as throws the KBD Heads onto a fire next to a skeleton of a giant dragon with three heads

S: May the gods of the Grim Underworld accept our offering and return this great beast to us

Sil'as and the rest of the Dagon'Hai begin to chant

All: Caldar, Nahudu, Camerinthum, Tarren, Rexus, Ater, Anguins, Ladon!

The skeleton will get up and flesh will fade onto it

Sil'as will look round and see you

S: Oh great Ladon these strangers would stop your rise to power, I beseech you, kill them!

Ladon will walk over to attack you, Aegis will attack Dahcsnu and the other Dagon'Hai will engage the Legeslatum in combat.

Ladon is level 200 and has the same attacks as the King Black Dragon. Once you defeat him there will be another cutscene.

S: Noooooooooooooooooooooooo!

K: Sil'as Dahcsnu, you are under arrest!

S: You can't lay a hand on me!

Sil'as will teleport away

K: Ah well [Player name], we'll get him soon, you can count on it

Congratulations! Quest Complete
You are awarded:
 * Ability to craft Draconic Sigils.
 * Access to Ladon's tomb (underneath the floor of the dirt place).
 * Ability to choose out of the following weapons: Tooth of Ladon (Sword), Claw of Ladon (Dagger), Horn of Ladon (Bow) or the Bone of Ladon (Staff) (you can switch these around by using your current weapon on the tombstone at the end of the tomb of Ladon) These are also dropped once each by Ladon as a super rare drop if you fight him again.
 * Ability to fight Ladon again every week.

Ladon's Tomb
A tri-layered maze filled with Dagon'Hai and Dragons, at the center of the bottom floor is the actual tomb of Ladon where you can switch the weapon you receive as a drop from Ladon.

Draconic Sigil
This can be created by using green dragonhide on a demonic sigil and using the wrapped sigil on a furnace, these can be used to summon Ladon (Rexus, Ater, Anguins, Ladon! is added on the end of the demon summoning chant), Elvarg (Regina, Viridis, Anguins, Elvarg!) and Flametongue (Rexus, Ater, Anguins, Flametongue!).

Trivia

 * While Guthix Sleeps is an unusual requirement for such a short and easy quest, this is possibly remedied by the boss fight at the end.
 * Ladon is a reference to the dragon with a hundred heads which guarded the golden apple tree in Greek mythology.