The Goblin War Chariot

From the introduction:

Barimoor is questing for immortality, and has set several plots in motion to finally bring his dreams to fruition. For the people in Karameikos this means big trouble. A rare deposit of the element known as Softstone has been found by Blackstone goblinoids living in the Black Peak Mountains, and Barimoor has contracted with them to retrieve the material. In exchange for magic items and other services the goblins have been mining the Softstone and sending it to Barimoor's underground realm. Unfortunately goblins are not hard workers, and now that they have their new magical toys they have begun using them to terrorize the countryside.