You must have either helmet equipped in order to enter the mountain cave.
You should fall down a floor, swim to the water vortex and you are another floor down. Go out of the water and follow the way up as far as you can go. Walk immediately north from where the last set of stairs take you.
you will end up at the top of the mountain, wear your boots of haste and run the full distance in order to make the leap.
Note that If you've lost your amulet and you talk to Jerom about your mission he will give you 200gp and will tell you that he got it.
the quest does not finish here, you still need to find the chief a 'Troll Lady'. If you check your Quest Log, "Troll Sabotage" quest will NOT be completed. you can read that Chief Grarkharok is still waiting for a troll lady. Maybe there's another reward if you can help him.
Jerom will tell you about his dream to open up a pet store. and about his parent's farm. he will also tell you about varieties of pets (spider,snake,dog,rat,cat etc)
Chief Grarkharok will tell you about his hobby of throwing stones at humans, and will also tell you about his love for snakes and frogs and will eat any dead frog or snake you will give him.
Jerom: Sorry, but I'm not in the best mood today. Player: Mission
Jerom: I'm not sure but I suppose that an evil troll lives in the mountains here! I saw him rummaging in the ruins of my house. ...
Jerom: I took a closer look and found my family casket ripped open. It contained a precious necklace. If I had it back, I could sell it and start over! ...
Jerom: Could you look for this mean beast, find out why he did and either get me some money ormy necklace to rebuild my business? Player: yes
Jerom: Gret! Finally, some light at the end of the tunnel! Come back and ask me for the quest when you solved this mystery!
Chief Grarkharok: Me Chief Grarkharok! No do nothing! Player: kill you
Chief Grarkharok: Hrhrhrhr! Me no fear of human! Me Chief Grarkharok!! Player: Lady
Chief Grarkharok: You help?? Human know troll lady for Grarkharok?? Player: Yes
Chief Grarkharok: What name of troll lady?? Player: Say the name of any female player on your world
Chief Grarkharok: Character Name ?!? Sound good! Bring troll lady to Grarkharok!! Here, give troll lady! Take take! Bring lady to Grarkharok for make tribe!! Now GO!
Jerom: Hello. Sorry, but I'm not in the best mood today. Player: Mission
Jerom: Thank you sooo much <sniffs>. Well, you know I lost everything, but recently I found this strange rope here. I don't need it, here take it!