I think the Relic of Isis is just a special loot. I don't recall having to do anything with it.
You should run into the Evil One as you explore the temple.
The door you mention cannot be opened until after you have raised and spoken to Anephas.
Have you been down the southeast passage from the room with Anephas' altar? (passage is blocked by a statue) Going down that passage and solving the door puzzle will give you access to the area where you can reach Kerah. When you reach Kerah, search her small world carefully - it's a one time only exploration area, once you leave, you cannot go back.
That should get moving!
Wyv
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