Judges 9:20
But if not, let fire come out from Abimelech, and devour the men of Shechem, and the house of Millo; and let fire come out from the men of Shechem, and from the house of Millo, and devour Abimelech. But if not, let fire come out from Abimelech, and devour the men of Shechem, and the house of Millo; and let fire come out from the men of Shechem, and from the house of Millo, and devour Abimelech.
Judges 9:20 is the closing curse and prophecy of Jotham's parable of the trees concerning the making of Abimelech as king. The trees holding a council, holding an election, speaking, acting like people, and making speeches are figurative personification, not a report of literal trees talking. The verse's language about fire and mutual destruction belongs to that figure.
The curse is conditional: if the men of Shechem and the house of Millo acted rightly in making Abimelech king there is no curse, but if they acted unjustly fire would come out from both sides so that Abimelech would devour them and they would devour Abimelech. That fire depicts their mutual ruin, foretelling the utter destruction of Shechem by Abimelech. The house of Millo represents the chief men among the people of Shechem.
Abimelech was the son of Gideon's maid who caused Gideon's sons, his own brothers, to be slain so that he might become ruler of Israel. Shechem was a very ancient place where Jacob stopped when he returned from Padan-aram to Canaan. The verse shows the degeneration of Gideon's family from trusting in unseen power to trusting instead in things that are seen.
The verse illustrates the folly of choosing such a murderous man as ruler. Ambition to rule over others shows lack of qualification for leadership, since those doing valuable work do not seek to exalt themselves over others. The mutual ruin by fire was the fitting consequence of alliance with the cruel, grasping Abimelech.