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. was at the time in Miami, dealing with a lawsuit brought against them by Miami Beach (because of damages caused by Zilla Junior while battling a Giant Bat). When Randy and Craven, who are staying in New York, get word of the fungus's attacks in Michigan, the two set out to handle the problem alone. However, the fungus attacks them and leaves them stranded without any food supplies or communication, so they must learn to survive without. Fortunately, H.E.A.T. manages to locate them. The millitary digs up the monster fungus and Zilla Junior throws it into a deserted area of sand dunes, where there are no food resources and the fungus dies. The team then learns that the lawsuit has been thrown out.
Armillaria is similar to Deathla, who has a mushroom form. The creature also may have had its tentacles based on the "Graboids" from "Tremors" from rising from the ground and grabbing objects.