The fight kiln is a revamped version of the Tzhaar fight caves. Before entering the Fight Kiln for the first time, a one-time payment of a fire cape is required. In the fight kiln, you must defeat 37 waves of monsters to obtain the rewards (your choice of an uncut onyx or the tokhaar-Kal which is an upgraded version of the firecape)
Basically it is a harder version of the fight caves where instead of your reward being a fire cape you can choose between an uncut onyx and a different firecape that gives +8 str instead of +4. The kiln is extremely difficult and will be hard to accomplish without good food and armor