DM's Carefully Balanced Encounter Destroyed In One Round By Spell He Forgot Existed

A Dungeon Master who spent three weeks designing a tactically challenging combat encounter watched it collapse in a single round after a player cast a spell the DM had forgotten existed, from a sourcebook the DM does not own, targeting a vulnerability the DM did not realize the monster had. The encounter, designed for the party's level 7 characters, featured a corrupted treant guardian in a narrow canyon, supported by four vine blight minions and a lair action that created difficult terrain each round. The DM, Patricia Action-Economy, had calculated the expected combat duration at six to eight rounds. Combat lasted one round. The party's druid, played by Neil Sourcebook, opened the encounter by casting Heat Metal on the treant. When the DM noted that the treant is not made of metal, Neil pointed out that the treant's stat block includes a metal collar that was part of its corrupted backstory, which the DM had added to the creature's description for narrative flavor. "I described the collar in the flavor text," Action-Economy admitted. "I said 'a rusted iron collar encircles its trunk, a remnant of its imprisonment.' I was trying to create atmosphere. Neil created an exploit." The Heat Metal spell, cast at 2nd level, dealt 2d8 fire damage and, because the treant has vulnerability to fire, the damage was doubled. The treant, which the DM had not given legendary resistances because it was "supposed to be a mid-difficulty encounter," took 28 fire damage on the first round and could not remove the collar without hands capable of fine manipulation. The party then focused fire. The vine blights were dispatched in a single Fireball. The treant fell in round one. The six-to-eight-round encounter took four minutes and twelve seconds of real time. "I don't own Xanathar's Guide," Action-Economy said. "I didn't know Heat Metal could target a worn object on a creature. I do now. I will never add decorative metal to a creature with fire vulnerability again. That collar was a creative writing exercise that cost me three weeks of preparation."