This video is really more of a "sneak peek" compared to Escape from Durnholde Keep. Seeing that the instance is tuned for level 70 players (mobs range from 68-72), my group of 67s was hard-pressed to keep up with the constant flow of enemies hellbent on killing Medivh. So, this footage contains as far as we were able to get before he died: a few waves of enemies, and the first boss in its entirety.
The video has much less narration than the previous one, due to the nature of the instance and the fact that it’s not yet complete. While Escape from Durnholde Keep is a large, sweeping campaign that takes place in a vast area, Opening the Dark Portal is much more contained. From what I could see, the sole focus of the instance is protecting Medivh while he works to open the portal. There is very little build-up to the start of the event, and little reasoning is given as to why Medivh expects you to protect him. However, I'm certain this is due to the instance not being finished, as Blizzard has said that it is a work-in-progress.