ESO Chronological Timeline Guide - Best Way to Play the Story

Markarth concludes the Dark Heart of Skyrim storyline, but before it does that, it will lead you across Skyrim to The Reach. Rally an unlikely group of defenders against the Gray Host threat, put an end to the Vampire Lord Rada al-Saran's diabolic plans, and conclude the year-long Dark Heart of Skyrim adventure. You can find the questgiver near Markarth, the primary city in the untamed Reach.

You can either travel to Markarth yourself and find the questgiver close to the Wayshrine, or pick it up from your Collections tab.

It will also set the stage for Blackwood and its story arc!