Rune dragon

Its scales seem to be made of runite.

Getting there

Open the teleport panel with ::teles, then the Slayer tab, and take the Rune dragons row. It is filed under Monsters.

Panel	Slayer - Monsters
Requirement	1 Slayer
Danger	Safe
Destination	Resolved when you click it - you take the one you are given

Tertiary
Item	Quantity	Rarity
Blood casket	1	1/1,500

Rates are per drop-table roll and are the same numbers the in-game Bestiary shows.