One: When you fly through the doorway towards the Rift, a movie
sequence will start, showing Rynn herocially lifting the RuneBlade
over her head before she and Arokh dive into the Rift. This is why I
said you cannot return to the lava valley above the Rift once you
have the RuneBlade.
Two: If you have ANY weapon other than the RuneBlade equipped, it
will *disappear* once you go through the Rift. You won't have it
anymore. So equip the RuneBlade before going through that doorway,
and save your game, as well. (I do not know if the patch to the game
addresses this or not).

Congratulations! You are now on your way to find your brother and
confront the evil Navaros.....