Seeing Lava is an achievement that requires the player to use a previously obtained fire cape on TzTok-Jad at the end of the TzHaar Fight Caves.
Be aware that you will run up to TzTok-Jad when using the fire cape on him, and that he can perform a melee attack when you are still within melee distance for his next attack.
Using a fire cape on a TokHaar-Jad in the Fight Kiln will not complete this achievement, nor can TokHaar-Kal capes from the Fight Kiln be used as a substitute.