I have this clue and this is where I have to go:

 

 

 

When you get tired of fighting, go deep, deep down until you need an antidote.

 

 

 

Yanille Agility Dungeon, downstairs. Go down into the area with the poison spiders and search the crate next to the stairs leading up. You must bring an antipoison potion with you, and 40 agility is required.

 

 

 

How do I get to the poison spiders?

 

 

 

EDIT

 

 

 

Wow right after I posted this I failed an agility obstacle and fell there.

If you pray at the altar it ports you down to the spiders.