Howdy.
Basic premise of this idea is that there is an interdimensional portal that connects numerous if not all of the planes together.
The Daemonheim Rift
The Daemonheim rift is housed at the bottom of Daemonheim, otherwise known as Floor 61, what Bilrach spent several hundreds year digging towards. This rift according to Bilrach's notes whispered to him and eventually became a voice as he got closer, only Zamorak prescence in Daemonheim seems to keep them at bay.
Mysterious Chronicle part 29:
Original message details are unavailable.
Year 137 of the Fifth Age:
His whispers have turned to voices, and I am close to Him now. I am but a few years from the end; yet, I fear the progress will be slow. So much of my power is spent defensively, propping up the floors that threaten to crash down upon me. The floors above are warped, out of control and dangerously unstable
This also leads a to conclusion that the energy from the rift is what causes the Warped floor distinctive Red colour. Though Bilrach is responsible for most of what we consider Daemonheim it is suggested that some of the Dungeons existed already before he arrived and were created by the Dragonkin.
Following Bilrach and Moia entering the Rift they were both transported to different Planes, Bilrach to Freneskae and Moia to Zamorak's location, this suggests the rift is more than capable of transporting to more than just a few worlds and as Bilrach was able to return from Freneskae it would imply there are other rifts through the Universe.
Bilrach in his convesation during Dishonour amoung Thieves:
Original message details are unavailable.
Bilrach: We were torn from each other as we tumbled through the dimensions. She would eventually find Zamorak, but to my knowledge she was gone.
Bilrach: I returned to Gielinor with the once-great Chelon-Mah captive.
Bilrach:I intended to use it to bring back Zamorak.
