Quest Talk:Passage to Darkness
This quest is actually quite easy to do solo. From Vindurhal, go down the steps and turn right. Except for a couple of mobs that you may immediately run into, there is no fighting the rest of the way. It is a mountainside trail with no turns until you get almost right to the Black Crack. You will come down an incline to a T intersection, turn left there and the Black Crack is right in front of you.
Here's the only trick: you only get credit for the quest if you come out of the Black Crack from inside Goblin Town, so go in, then turn around and come right back out to achieve the quest objective.
If you happen to be doing the Goblin Fire quest as well, I like to combine these two. After exiting the Black Crack, go back up the incline and jump down to the goblin camp below. There's a goblin fire-pot right there and you can get the rest through a nice linear progression working the camps back the direction you originally came. This area is much easier to manage crowd control in (especially solo) than coming in the front door down below.