Trouble farming the UW

Okay, so i was watching a video on Youtube the other day, a sin solo farming the UW. It looked interesting, so i decided to give it a try. I pull together a build, and make my way to the Underworld. After taking the quest and using my Perma skills, i go off in the same direction as the guy in the video, only to find that the door he went through is closed to me.

So what the heck? Am i missing something here? Any help is appreciated!

The video:

http://www.youtube.com/watch?v=Rc4saH6Rfe8

The door im talking about is at :31 (31 seconds)

Probably missed a grasping darkness.There are a couple that patrol.

so i have to kill all the grasping darkness'? okay, thanks a dozen!

Yup, after taking the quest called Clear the Chamber from the Lost Soul, you need to kill three groups of grasping darknesses in order for the doors to open. Should be around 3 or 4 graspings in each group.