What it does
Mystical Fire is a Fire-type special move that deals damage and always lowers the target's Special Attack by one stage — it is 100% guaranteed, exactly like Icy Wind's speed drop. Every time Mystical Fire hits, the target's Special Attack drops by one stage.
Fire-type coverage in doubles targets real threats. Steel, Grass, Bug, and Ice types are all weak to Fire, and Steel-types in particular appear frequently in high-level doubles play. Mystical Fire gives both the Fire-type hit and the possibility of reducing the opponent's special output, which can matter when you're facing a Pokémon that's threatening to deal significant special damage to your team.
This move is special category, calculated off Special Attack — it belongs on Pokémon that already invest there.
Single target, not a spread move. You pick one Pokémon per use.
Who runs it
Hatterene (8%) is a Psychic/Fairy type with high Special Attack, typically found in Trick Room squads because of its low Speed. Mystical Fire gives it Fire coverage for Steel and Grass types that would otherwise wall it, and the Special Attack drop has direct value in Trick Room: if you clip an opposing special attacker's Sp. Atk, your team takes less special damage for the rest of that Pokémon's time on the field.
Alcremie (17%) is a Fairy-type special attacker that can function inside Trick Room as well. Mystical Fire handles Steel types that resist its Fairy STAB while adding the chance to weaken opposing special output. Steel types come up often enough in doubles that having a Fire move is more than just coverage insurance.
How to use it
The clearest target for Mystical Fire is a Steel or Grass type that your Fairy or Psychic STAB moves cannot touch efficiently. The Special Attack drop is a secondary consideration — don't plan an entire turn around it triggering, but always appreciate when it does.
In a Trick Room context, Hatterene and Alcremie are slow attackers going last. Each turn, you're choosing between your available attacking moves — use Mystical Fire when the target is weak to Fire or when the opponent's Special Attack is something you want to reduce.
Against a Steel type: reliable 2x damage, possible stat drop. That's a solid use of the move slot in a single action.
Tips & strategy
The Special Attack drop's value compounds slightly over time. If you're in a longer game and the same special attacker stays on the field, even a one-stage drop reduces its damage output every turn. Your partner takes less damage. Small margins like this show up more in slow Trick Room sequences than in faster matchups.
Flash Fire completely negates the move — the target absorbs it and gains a Fire-type power boost instead. Check team preview for Flash Fire users and redirect the Fire move elsewhere if you see one.
For Hatterene specifically, the decision between Mystical Fire and other Fire-type options often comes down to what else the base power and the secondary effect comparison gives you. The stat drop possibility makes Mystical Fire worth considering over higher-power alternatives when your team would benefit from suppressing a specific special threat rather than just raw damage output.