If you can cast Vanish then you can
One possibility is Vanishing through the closed barrier at the Oracles. But beware! There seems to be no way back from Ishad N'Ha in this case! At least, I wasn't able to Vanish back [img]graemlins/hehe.gif[/img]
Another possibility is Vanishing through the underwater grate at Rephtian. This way, you can skip the Crypt almost completely and reach the Gold Key and Nivius still. The advantage of this method is that you will have the signet ring so that you can make the Oracles to open the way to Isnad N'Ha.
If you cannot cast Vanish then you have to complete the Crypt. There is no side way (at least, no side way known to me) to reach Ishad N'ha in this case.