That exit which appears to go back to the Ishad N'Ha plain actually doesn't. Follow the path all the way and you will be in the area for Brimloch Roon.
Wyv
P.S. actually you can use that path to return to the plains by jumping off but since that isn't what you want to do, just follow the pathway.

I've the Mavin sword and I've done all quest in Ishad N'Ha and Valea

I'm in front of the dragon in his cave and I don't find any exit which lead to Brimloch Roon?!

The only exit get me back in the Ishad N'Ha plain

There is a closed door in the cave, how to open it??

Ok thnx