(1) The Blackmarsh exudes creepiness, and there is something supernatural in the air. Nathaniel warns of this if you interact with the Blackmarsh tree a few paces up the path on the left. Beware of the nearby marsh wolves that bear down on you as you approach them feasting on some unlucky critter.
As with all wolf encounters in the marsh, AoE stunning abilities such as Repulsion Field keep them at bay. Pay attention to Group Heal because everyone will likely take damage simultaneously as the creatures nip away.
As you explore the Blackmarsh, you come across tears in the Veil. These Veil tears block your passage in the physical world. If you attempt the "Tears in the Veil" side quest, you can close these tears once you reach the Fade on the Blackmarsh Undying map.
(2) Fight through more wolves at the first intersection. If you take the left (north) fork, the path leads to a dead end at one of the sealed Veil tears. Take the right (east) fork to continue on the main quest to find Kristoff.
(3) Go right (east) and you'll see a darkspawn corpse in front of you after you take on a group of blighted werewolves. Slay the werewolves and interact with the darkspawn corpse. You surmise that Kristoff killed the darkspawn, so he must be close.
(4) After combing through the ruins for loot, side quest items, and codex entries, head north out of the broken town and deeper into the marshes. (You can't get through the locked town gates to the east at this point.) There are many enemy ambushes in the area, so be on your toes.
(5) In the middle of the northern section of the marsh, you find Kristoff's camp. You can tell from more dead darkspawn and Kristoff's cot that he must be very close now. Recover Kristoff's Locket, a gift, in the rock pile near the cot.
(6) Unfortunately, Kristoff was slain by the very darkspawn he tracked. His body lies in the map's far northeast corner.
Kristoff's body is a darkspawn trap. Once you interact with it, your party gets transported to the Fade. Make sure you accomplish everything you wanted to in the Blackmarsh before touching Kristoff's body or it will be a long time before you return.
When you interact with Kristoff's body, darkspawn surround you. It's a trap meant to slay any Grey Wardens who followed Kristoff. The First, a loyal servant of the mysterious Mother, casts a powerful spell that rips a tear in the Veil and transports your party to the Fade. You must now battle through the Blackmarsh Undying to return to the real world.