From GuildWiki

After Rurik tells you to open the drawbridge, you'll find the NPC for the bonus while crossing the tar. Make sure you FOLLOW the Ember Bearers until they open the door or you won't be able to complete the bonus. If they spot you just run away and they will go back to walking. (Sometimes they get stuck and the door doesn't open... if you lure them away from the door a few times they usually get unstuck and open the door). Go through the door that they have opened and kill the Flame Keepers to receive the bonus.