As a 2AP action, you may select up to (8) hexes within 6m and LoS - so long as every hex touches another. Choose (1) of those hexes to be the Crit! area.

2AP, Focus, 6m
All hexes burst into flames. All beings within must pass a DEX save or take a (✦) + SPR of Fire damage, or (2✦✦) + SPR in the Crit! area. On success, the damage is halved. If a being is surrounded by Fire Zone hexes on all sides, they must make a FOR save or take a level of Exhaustion.
The effects of this Zone repeat at the start of your next turn, if you choose to spend an (EP⇧). Otherwise, Zone ends.
You may end the Zone as a Free Action at any point during the round. When the Fire Zone ends, the hexes become Dangerous Terrain until the end of combat.