There was a tiefling living in the elven burial chamber at the Standing Stones. It (he? she?) was pulled through the rift before it collapsed, and it seemed that the rift took something from it's dying body when that happened.
Per the journal we found there, this tiefling was lured and seduced by the power of the creature Hravja. I think this may be the same creature that attacked you
The final journal entry:
I think the rift took back the part of the tiefling's soul that it had sacrificed to open the rift? This may have caused the rift to collapse."My fear at giving up a part of my soul to the portal was completely without merit. Who cares if I can’t remember some details of my childhood? What are memories compared to tangible Power? The part of my soul that I gave up wasn’t even very large, certainly not like that required to open a portal in the first place!"
Anyway, this info, paired with the recent notes that past-Jareth had written, lead me to believe the following: the PCs (the Master's Lieutenants) all sacrificed part of their souls to open the portal to the fire lands in order to rescue this Gweyndolyn from the clutches of the Fire Lord (an Efreeti lord). That mission failed. This missing part of our soul accounts for some of our memory loss. I am also thinking that our sacrifice may be involved in our recent mysterious transportation to the Plane of Water. Maybe the portal re-opened?
Our previous selves also seemed to grow colder and more ruthless as we matured - this emotional regression may be due to losing part of our souls.