The Dark mage is located at the centre of the Abyss. He can repair damaged pouches for free and give back an Abyssal book and/or Small pouch to the player if lost. It's not required to bring him the pouches, because he can repair them even while they are in the player's bank. An alternative is to contact him with the lunar NPC Contact spell however this spell will only work if the player has met the Dark mage beforehand.
He was one of the Z.M.I. initiates charged with casting teleportation spells until they happened to fail and end up in the Abyss again.[1] He was the first to do so, and his superiors kept him there so he could hold a portal spell open so other mages could go to the Abyss through it.[2] If talked to, he will offer a dire set of consequences if he fails in this task (including head explosion, if lucky, or universe explosion, if not-so-lucky).
He can stay in the Abyss for a long period of time because time in the Abyss passes slowly compared to the time in Gielinor, so he doesn't need to eat.[3]
In the final cutscene of The Temple at Senntisten, the Dark Mage is seen to nearly lose control of the portal due to the shockwave following the creation of the portal in the temple under the Varrock Dig Site, but he manages to regain control just in time, commenting that he had just prevented the destruction of the universe.
^Abyssal book, "Enter the Abyss", RuneScape. "We took a large number of initiates... We then had them repeatedly teleport... seeking to replicate the original error that caused the first entry into Abyssal Space. Once one of our initiates had managed to ‘fail’ his teleport and appear in Abyssal Space"
^Abyssal book, "Enter the Abyss", RuneScape. "He was then charged with remaining there and holding a portal spell open, so that the more senior members of the Z.M.I. could gain entry via his portal."
^Abyssal book, "Enter the Abyss", RuneScape. "The slow passage of time within Abyssal Space means we don’t need to worry about feeding him or anything."
