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Here is a fast and somewhat safe way to get to Silithus from Desolace.
Fly to Nijel's Point.
If you've never been to Nijel's Point, just go to Ratchet and walk west through the Barrens and continue west after you reach the Crossroads.
Alternatively, you can fly to Astrannar and travel south and a little east until you reach the Talondeep Path. Walk through the Talondeep path (which is mobless) until you reach the Windshear Crag in the Stonetalon Mountains. From there, travel southwest until you reach the Webwinder Path (on the way, watch out for numerous hostile goblin mobs).
Walk north through Stonetalon Mountains following the road; go west when you reach a westward pass in Mirkfallon Lake (I believe it is nearby some fire elementals). You might want to get your flight path at Stonetalon Peak on the way, it's to the far northwest. Don't worry, the mobs aren't too tough here (between 20 and 26, if I remember correctly). If you are having trouble with the mobs following the main pathway to the Charred Vale (which holds the entrance to Desolace), there is an unmarked and mobless path on the west side of the road between the entrance to Sun Rock Retreat (a Horde town) and the entrance to Mirkfallon Lake. This less-traveled path not only gives you a safe passage to the Charred Vale, but also gives you an excellent bird's-eye view of Sun Rock Retreat as well.
Cross the entire Desolace map, going south... This is probably the most painful part of the journey, but if you were killing skeletons for the Scarlet Monastery quest down south, you're not far. Get into Feralas (or you can go to the far southwest coast of Desolace and swim to Feathermoon Stronghold, but watch out for the occasional underwater mob).
You must be very cautious here, since the mobs are likely quite strong for you (around 40+), so try to avoid them as much as possible. Just follow the road going south or simply swim along the shore, although this method takes a while for non-druids. You may want to take the Feathermoon flight path on the way there, since you'll be passing by.
As soon as you have your flight path, you'll be swimming along the shores of Feralas all the way down south. As soon as it says you're in Silithus (your channels will change), drown yourself and spirit rez in Cenarion Hold.
You are now in Silithus! Don't forget to get your flight path!