From the RuneScape Wiki, the wiki for all things RuneScape
A Mithril grapple is used to grapple with Agility onto objects such as Strong trees and Rocks, or even scale walls. It's wielded in the arrows slot and used with a mithril crossbow (also a Dorgeshuun crossbow or any metal crossbow higher than mithril). The grapple can also be fired from Karil's crossbow.
Sometimes, the grapple may break (though this is rare), so it is a good idea to have more than one in your inventory (only one may be wielded at a time). When a grapple breaks, the following message appears in the player's chat interface: "After untying the rope, you find that your grapple is broken and useless".
There is a respawn in a crate over the agility shortcut in God Wars Dungeon. Drop your first one, then search the crate.
Producing a grapple
A player Grappling the wall in Yanille.
Please be aware that you will need a crossbow along with the grapple to perform these agility maneuvers. This used to require a mithril crossbow or better (dorgeshuun crossbow or Zanik's crossbow would also work,) but has been tested to work with a bronze and blurite crossbow. (6 December, 2009)