The first and foremost issue would be explaining how the Abyss is even leaking into our world. Canonically, the barrier between the Abyss and a given plane is fairly resilient. It's not too difficult to claw your way between a plane and the Abyss and back again (though that's in part thanks to a demonic ritual), but things outside of the travelers and what they carry with them tend to stay on their side of the divide.
Note, I used the word 'tend.' So far, we have but one example of the Abyss continuously leaking into Gielinor: the Anyssal Demon area of the Slayer Tower. There, we can observe a portal that presumably leads directly into the Abyss and has part of the Abyss creeping out of it. The portal, however, appears to be fairly self-contained (leading me to believe it was probably a controlled summoning rather than something naturally occurring.
I suppose the easiest way to explain why the Abyss is leaking into Gielinor is to take the stance that the Abyss itself isn't leaking in. Rather, it could be that some part of Hostilius's body (the fleshy parts of the Abyss we actually get to visit) is trying to pull itself into our reality. Given Hostilius's apparent death, this could be some reflexive reaction to sensing the life flourishing on Gielinor (no consciousness removes the Chthonian inhibition to consume).
You could explain the widespread pervasiveness of this dungeon by having this part of Hostilius attempt to pull itself into our world by using the equally widespread runecrafting rifts (the ones the altars were built on) as veritable tentacle-holds (thereby explaining the presence of the rifts in the Abyss).
As for your artifact, why not make it some chunk of Hostilius's brain/heart that hasn't quite gotten the memo that he's dead yet?
28-Oct-2016 22:36:04