An Honored Guest

 is a quest in. It activates automatically after the DLC is activated and the player enters the world map.

Synopsis
High Harbinger Vatnir, leader of a Rymrgand cult out in The Dead Floe, has invited the Watcher to a feast in their honor, so that they may honor Rymrgand impending dominion over Eora. The Watcher decides to come.

Walkthrough

 * The quest is activated by the wax-sealed letter that's automatically delivered to you once on the world map. If you've just started the game, this occurs after finishing Stranded upon leaving Vilario's Rest on your ship.
 * To begin, simply head south-east, past the Windless Waste, to the newly-marked Harbinger's Watch. After a run-in with an excited Harbinger, proceed to Harbingers' Retreat in the center of the map.
 * Inside, you are greeted by Harbinger Valbrendhür, who gives out a free set of Harbinger robes. Your goal, Vatnir, is in the northwest. He will introduce you as the Duskspeaker, allowing you to react in a variety of ways, including a special option for Clergy with Deception 8.
 * Then he will proceed by listing your accomplishments, including a rather, uh, skewed view of the events of, up to and including pinning the Hollowborn curse on the Watcher. Perfect. You can reply with a special option, Religion 15, or just go ahead and skip to the feast. Except there shall be no feast, as something called The Messenger appears in the Watch, leading to Vatnir's cultists tripping over each other on the way out the door.
 * Exit the temple after this peculiar introduction to be introduced to an even more interesting character: A dracolich. After several strafing runs over the village, killing a dozen or so Harbingers - much to their delight - the dragon will land and await your approach. Due to its state, the dragon can't speak - or doesn't want to - and will attack after a short exchange. After dispatching it, head back into the Retreat.
 * Vatnir can be found cowering in his secret room, accessed through his quarters and a hidden switch in the skull. After talking to him, you'll learn that Vatnir basically invented the Messenger as a way to explain the dragon's raids on the Watch - and isn't actually the fanatical zealot he appears to be. He will suggest looking for answers in the The Rimebound Temple, which the Dead Floe grew around. You can obtain Vatnir as a companion with 12 Deception, 9 Streetwise, 11 Religion or 11 Metaphysics, or tell him that he has to set things right. Either way, Harbinger Hafjórn will appear, and after exchanging a couple of perplexed lines with Vatnir, get an untimely promotion to deputy High Harbinger.