"Murkmire" is a new part of the story DLC that comes to The Elder Scrolls Online, where players can visit part of the Black Marsh, home of the Argon Lizardmen. In the above video, jump to about 13:30 mark and you will be able to see what that looks like in action. The new DLC contains the settlement of Lilmoth, which will be familiar to the readers of Greg Keyes' Elder Scrolls novels, looking as swampy as you wish.
Chris Livingston wrote a demo of Murkmire, which he had seen at the Quakecon, where creative director Rich Lambert described it as "about fifteen to twenty hours of content with new stories, there were dailies, two bosses, a new dungeon that goes with it."
In addition to the ESO Plus members and the in-game store, Murkmire will be "available as a daily reward for the month of November," reads this blog post.