where is the location ?

42,5 Clear north in center for those w/o coords.

Have just soloed as 42 Rogue, thought I'd leave comments for those who want to do same.
Overall, not too difficult, though hard to say for other classes, since by stealthing I was able to move quickly past the lower level guys.
There are a few things I haven't seen anyone mention, so I'll note them:
The Kurzen Elite, despite the name, are regular mobs who die easily. Dispatch at will.
The Shadow Hunters are more dangerous. If you aggro one while fighting someone else, they will shoot you from afar, probably with their ridiculous poison that does 38 nature damage per second. Watch out for that. They are fragile, so engage them first when possible.
The Witch Doctors are shamans and don't pose much threat. They drop fire totems that do nothing. Bleh.
The Subchiefs are weird. Sometimes they roll over, other times they mysteriously took me near death. They occasionally deploy a red aura around themselves. If you see this, back away it does dps to you if you are near it quite quickly. they can't move when using it, so just back off until it goes away. Other than that, they die fairly quickly.
Finally Kurzen himself: the room he is in has several subchiefs and a hunter in the corner, near him. There is little chance of fighting Kurzen 1v1, as one subchief stands near him at all times. The others can be pulled, though I don't know how other classes would handle the hunter without aggroing Kurzen. For rogues, just kill all others and go to the back corner stealthed, wait for the hunter to get sufficiently far from Kurzen, and kill him. Make sure to use your Crippling Poison in here (but you should know that by now, right?). Here's how I killed Kurzen:
Stealth, Cheap Shot, Blade Flurry, Evasion, Riposte when possible. I ignored the subchief with him and concentrated on Kurzen, the Blade Flurry pretty much killed subchief without my needing to actually target him (love that talent!). Something you need to know about Kurzen that only 1 other poster has seen fit to mention:
KURZEN CAN VANISH. Took me by surprise. Twice he vanished on me. The first time was ok, gave me a chance to finish off the subchief. 2nd time was annoying cause he was almost dead. Anyway, be aware of that, he'll disappear and try to run off.
The quest reward is pure souveneir, as usual, but vendors for 80s. The exp reward is quite good, and kurzen himself gave me about 575 exp. Not bad. A lot of fun and a real challenge to your soloing skills, as the mobs, while kind of weak, are numerous and close together. Enjoy yourselves!

i just soloed this quest as a 44 affliction specced lock...tried to solo it at 40 but couldn't. It was difficult even at this level and i still died once. I just thought i would add a few comments that may make it easier for some others.

First after you go into the cave stay to your right all the way down. Watch out for the stealthed NPC's from the entrance to the bottom of the first long ramp. There are alcoves to the left which are great if you just want extra xp and the occasional level 30 green drop. And beware almost all these NPC's will try to run when health is low and if you let them get too far away, they will pull aggro to you.

The fel hunter works great with the witch doctors and headshrinkers becasue they heal themselves and shoot shadowbolts at you. The fel can negate their magic effects.

The Kurzen elite have some type of red aura that afflicts you with either a DOT or a debuff. But they are just regular NPC's not "Elites".

There is a treasure chest about 3/4 of the way down on the right on a platform behind some crates. I snuck around the back side against the wall and killed the mobs up on the platform first, looted the chest, and then went back to the lower level and killed the rest. There are quite a few NPC's in this area and I pulled two several times. Kill everything in this area and beyond to Kurzen because the respawn rate is incredibly fast.

The shadow hunters have ranged attack so i just put my VW on passive and stay and then ran up to them and stayed too close for them to shoot ranged attacks, and pulled them slowly back to my VW and had him attack. I didn't pull any extra aggro this way. Just be careful not to stay too far away from your target as the respawn rate for this area is quite fast.

After the ledge, stay to your right and go around the rock and there is a mine along the right wall and another chest to the left by some crates. There are several practice targets in this area and several NPC's. Again pull carefully to avoid major aggro.

DO NOT use fear or howl until you have already killed everything anywhere remotely close to you....hence how I died.....i ended up with like 10 NPC's on me at once in this room.

After I got the above mentioned chest, then there are appx 5 more NPC's to the right in a pathway guarding the path to Kurzen. The first three you can pull solo but the next two are side by side and you will pull both. I just set my Succubus on attack and she seduced one while we killed the other.

Finally in the room where Kurzen is (Very end of cave on right, there were 5 more NPC's (the same Kurzen Elites from above that aren't). Three are roaming about and one is standing on both sides of Kurzen. He is heavily armored and vanishes every 20 seconds or so. First I *****"hugged the wall"***** and waited for each of the three roaming NPC's to get the furtherest distance from Kurzen and the others and got right on him and attacked with VW. If you attack them from the middle of the room, you will aggro all of them. If you have any kind of stun or finishing move that will keep him from running at low health, use it. I used shadowburn and deathcoil the instant they started to run.
The NPC that is closest to Kurzen (back left corner)is tough to pull, but I did not pull any aggro by attacking him when he was furtherest to the southwest(sort of to the front of the table) Kurzen is standing behind and again I was hugging the wall. A friend said he tried to pull him from the area to the nw (sort of behind)the table and aggroed Kurzen and the other 2 as well and died.

Finally just Kurzen and two guards left. I pulled my succubus and sent her to attack on one of the gaurds. I immeidately DOT'd Kurzen. She aggroed all three, seduced one of the guards and was fighting Kurzen and the other guard. I DoT'd the second guard and pulled aggro away from her with searing pain and SB then feared him and left her fighting Kurzen. Kurzen vanished and she then attacked and we finished off the second guard. I DOT'd the first guard, life tap, health funnel and Kurzen reappeared. and I feared him. Used mana potion. Spammed 2-3 shadowbolts and first guard was down. Succubus ran after Kurzen but was almost dead. Kurzen vanished again and I used healthstone and summoned VW. Kurzen reappeared and attacked VW, I use CoElements and hit him with searing pain x 2, and bandaged (runecloth), then wanded (fire). He again disappeared, life tap x 2-3, health funneled VW. I feared Kurzen again as soon as he reappeared and then VW attacked. I drained life, then DOT'd again the ones that had worn off, SB and Shadow burn. The end.......it was a very long fight, and I ended up with about 1/4 life and 1/4 mana left. And my VW was almost dead at the end. Then I hearthed out to BB. The respawns were already starting to appear.

Also everytime Kurzen disappears you have to retarget him.

Hope this will help. But I would definitely advise you to have at least one other person and some health potions, mana potions and bandages with you.

It was difficult to solo but doable.

For me this quest was tougher that it first appeared. At level 40 you can probably solo your way close to Colonel Kurzen, but to take him down you will need to deal with the Colonel himself (40 elite) and his two body guards (38) at the same time. So either go in with a party or brush up on your crowd control!