Hi bluemonkmn!
Haha!!! It's great fun to answers questions about my game. I love it!

The last step to complete the Hylden Portal objective is to close the portal in the whirlpool at the lake of the dead. You got the first part right: backtrack to the Hylden Fortress and use the portal to reach Vorador's Mansion. Once you're back in the mansion, make your way to the lake in the Pillars of Nosgoth area. When you get near the cliff, Kain will close the portal automatically by throwing the Nexus Stone in the lake.

Also, as a friendly warning, this means that when you close the portal in the lake you cannot go back into the Hylden Dimension or Hylden Fortress, they are closed forever. If you still want to complete other objectives or find hidden items, you have to get to them before you close the portal.
You must be near the end! From what I heard, this is the last objective most people deal with. In what order did you complete the objectives?
Have fun!
