Goblin fire is generally considered the most dangerous of the various kinds of Goblin munitions, as it is the only one with any persistent effects. Goblinfire creates green flames, which cannot be put out by any known means. These flames will naturally spread for a period of seven days before going out, and are notable for the fact that they can consume magic as well as physical materials, making them very difficult to contain by supernatural means.
Though goblinfire can spread across any known material, it spreads most quickly on flammable materials and very slowly on non-flammable or wet materials. Sappers memorize a chart detailing the rough speed of spread across a variety of materials, and Catherine at one point notes that ordinary mud is near the bottom, indicating that it inhibits the spread considerably.
Goblin fire is most commonly used as an area denial weapon, since any ground claimed by it cannot be entered until it extinguishes, and anyone touched by the flames can only be saved by quickly and carefully severing any flaming limbs before retreating. It can also be used to cut off avenues of retreat or destroy forces incapable of retreating. It is also useful for threatening magically protected areas because most magic will only cause it to spread faster, even spells intended to protect the area.
William of Greenbury used goblin fire in the city of Summerholm in order to start a major fire. His hope was that Wekesa would be forced to inflict backlash on himself to remove the enchantments on his mage tower or else allow the entire city to be destroyed when the goblin fire consumed the vast amounts of magic and exploded. The plan succeeded in weakening Wekesa, but he was still strong enough to kill the Bumbling Conjurer and escape with his life.
During the Second Battle of Liesse, Catherine used goblin fire to destroy Akua's throne room, cutting off all avenues of escape before leaving through a Faery Gate. However, Akua was able to force the gate to remain open through the use of an Aspect long enough for her to escape and capture Catherine.
In the battle for the Red Flower Vales, the Witch of the Woods is able to eliminate a goblinfire attack by using magic to accelerate time until the goblinfire burns itself out. This is the only time in the story that goblinfire is defeated by magic.
The exact system used to create goblin fire, or any other variety of goblin munitions, is unknown. However, it is believed by many in the know that the process involves summoning and killing Demons in order to use their remains as alchemical reagents.
Upon seeing the sappers' chart of spread rates, Masego noted that some of them showed ratios of occult significance, though he did not elaborate. This may have been foreshadowing to the later revelation that goblin fire is made with the remains of demons.