after finding the nivius tomb and getting the ring, i thought, the way through the grates is free, but it is NOT!
now i'm searching for two days to get out of that **** dungeon!!! but i couldnt find any way.
if anybody could help me to find out, i'll really be very glad.

You will kick yourself for missing it. There is a lever underwater on the left wall at the left grate. This takes you back to the ratpie dude. From there it's the elevator Of Flokis'Ra (and its' secret room) and out to the crypt entrance.