1) Talk to Denulth, who's located by the tents south of Burthorpe castle. Denulth will tell you that this troops need a new way of entering Death Plateu, as the trolls have begun to get more stronger each day.
To do so, you'll need to find a dwarf located North-West of Burthorpe.
2) Head North-West to enter Death Plateau.
WARNING: Trolls and Burthorpe soldiers roam here and fight each other. If enough soldiers die, the trolls could turn on you.
Once in Death Plateau, head to the cave just north of you. Once inside, talk to the dwarf. He'll give you information on the premise of getting the trolls to 'shut-up' from stomping. He'll tell you to talk to Sherpa, a man who lives South-West of here, who knows of a secret path.
3) Traverse pass the soldiers and trolls, but turn left when you hit the T junction in the path and head south up the ramp. WARNING: Continuing on will cause you to walk into deadlier trolls.
Once up the ramp, continue south and head up another ramp to find the small house.
Enter and talk to Sherpa
4) Sherpa will tell you the alternative path if you bring him some much needed supplies. The supplies he needs are an iron bar to repair his climbing boots, 10 trout and 10 bread.
Take the boots and an iron bar to the smithy in Burthorpe (South-West of Denulth), who'll repair the boots free of charge.
Take the boots and 10 unnoted cooked trout and bread back to Sherpa, who will tell you the alternative path.
5) Head out the north door (not the one you came through), to enter Death Plateau. Follow the small path along the Cliffside to see the overview of Death Plateau. To trigger the next quest step, you'll have to find Sherpa's easel, which is located at the end of the path (until you meet some fallen rocks, not hard to miss). Click the easel to trigger the next quest step.
6) Once you've clicked the easel, return to Denulth and hand him a painting of Death Plateau (won't be an item in your inventory), who will thank you for your hard work.
Congratulations, quest complete!