How to reach Grelleth's Palace
Unless you have completed the Portal Attunement quest for Grelleth's Palace you'll have to take the long route to this zone. Grelleth's Palace is one of the most difficult RoF zones for you to reach as it requires you to travel through a few different VoA zones - one of which that's difficult to navigate as it has see invis enemies.
Since we're traveling through mostly VoA content you'll want to start in Argath - to reach Argath take the Guild Hall portal. The picture above shows you the upper half of Argath, in order to reach the lower part that has the Valley of Lunanyn zone line you'll need to travel down the staircase that's marked with a blue circle.
Once you're on the middle floor of Argath (shown in the picture below) you'll be able to reach Valley of Lunanyn which is marked with a slightly larger blue circle on the map below. The little blue arrow points to the tunnel you need to take in order to reach the zone line.
After arriving in the Valley of Lunanyn you're going to want to invis up if you haven't already and run all the way south to the Sarith zone line. If you're worried about being hit I'd recommend you follow the western most wall instead of going straight down the center of the zone.
Sarith, City of Tides has zone lines to a bunch of different VoA zones - all a short run from where we are. The zone we're going to is Resplendent Temple which is on the other side of the waterfall up north. Avoid the "war beasts" that you see up here as they see through invis. Just hug the wall.
Once in Resplendent Temple... Things just get even more risky. From the zone line you'll want to follow the blue line in order to reach the tunnel that houses the Grelleth's Palace Fear Crystal. There aren't any enemies you'll have to worry about while running here - the hard part is coming up next.
In this tunnel you're going to encounter some zombies which see throuh regular invisibility. You can hop skotch through the tunnel with regular invis but you'll need to be very careful and wait for zombies to path out of your way if they're in your way.
Hug the walls as you run through the tunnel, like my line shows on the map below and don't be afraid of training the Fear Crystal. It's unlikely anyone is right there - this isn't that good of a camp location or anything like that.