If you are planning to do this solo, using a weapon with a high melee bonus comes in handy when running through. The Mukkir all ignore shields but do use War Magic, so wielding a magic absorbing shield such as the Sanguinary Aegis will help.
From drop go down, and hang left until a room with pillars, and multiple exits.
Take the West exit and stick to the right wall, there will be one dead end but just turn around and keep sticking to the right wall until you enter a 4 way intersection heading north. Continue straight north (the west and east tunnels connect to each other and take you right back to the intersection).
You'll come to some bridge like ledges over a pit. Go due north and once out of the pit room stick right and go down a few levels, continue until you go up a level.
At next intersection, go left (W), next one right (S), then left (W). You will be in a corridor with windows looking into a very large room. Go into the room and take the W exit.
You will find an unlocked gate. Go through, and stick right.
There will be one last Barbaric Mukkir in the (circular) hallway before the room with Guardian. If you solo this, and have been running through, it might be an idea to kill this Mukkir in the hallway away from the room, so it doesn't come to the Guardian's aid (and does not lure the Mukkir on the other side of the circular hallway).
If you are ranged or a mage, you can jump on the frame of the door, and the Guardian will not be able to reach you with melee attacks and rings, making him an easily kill.
After killing the Guardian and opening the door, it is a relatively short run, generally N, past a few Undead Ruschk. Note that the door closes very quickly.
One last Mukkir before the Dark Crystal Spire. You can kill it or, in the room with the Spire, there is a hallway to the left with a surface portal at the end. If you are ranged/mage, you can attack the Spire without any Mukkir coming into the room from this hallway. Archers can go full speed (the Spire has no missile def). Mages do best using walls (especially Tusker Fist).
The portal spawned by the Spire needs to be double clicked to be used.
Lanaith tells you, "As was promised, my Master has opened the path to the Dark Isle. However, the Mukkir have reacted quickly, raising Wards about the Isle to stop us from gaining access to it."
Lanaith tells you, "My Master has located a nest of Mukkir on Vissidal Isle, near that village of strange Merwarts. He believes they are the ones holding the Wards in place. Go there, find a way to bypass their Wards, gain entry to the Dark Isle."