Im sorry if I make this difficult for you, but if you open the co-ord thing on the guide for treasure trails http://www.tip.it/runescape/?page=treasure_trails.htm#coord, then put in these co-ords:

 

 

 

 

 

 

 

01 Deg 24 Min North

 

 

 

08 Deg 05 Min West

 

 

 

 

 

 

 

I don't even know how I am going to get there, do I have to do a quest?

Yea, you have to start "Regicide"

Do I have to go right through the underground pass or what?

Yes. To start Regicide, you need at least level 56 agility and you need to have completed Underground Pass.