Armillaria was a giant, mutant fungus which had miles of tentacles. It attacked and destroyed farmland in Michigan. Several members of H.E.A.T. were in Miami at the time, dealing with a lawsuit brought against them by Miami Beach (because of damages caused by Godzilla while battling the Giant Bat). When Randy and Craven, who were staying in New York, got word of the fungus' attacks in Michigan, the two set out to handle the problem alone. However, Armillaria attacked them and left them stranded without any food, supplies or communication. Fortunately, H.E.A.T. managed to locate them. The military dug up Armillaria and Godzilla threw it into a deserted area of sand dunes. Without any food sources, Armillaria finally died. H.E.A.T. later learned that the lawsuit against them had been thrown out.
- Armillaria is similar to the cancelled kaiju Deathla, who has a mushroom form.
- Armillaria's tentacles may be based on the Graboids from Tremors when rising from the ground and grabbing objects.
- Armillaria shares some similarities with the Toho kaiju Matango, being a mutant mushroom.