Here's some rules I came-up with for using flaming arrows:

Common or longbow archers can fire flaming arrows when this rule is in effect. Each Lore symbol rolled allows the attacker to roll two additional dice to cause damage (so, if you rolled two Lore symbols, you would roll four additional dice. If these additional dice are Lore symbols, they have no effect). Archers with flaming arrows can target a hex with a wooden bridge, and on any sword on shield (SoS) or Lore symbol, the bridge is set ablaze. It becomes immediately impassable and is removed from play. If a unit is on the bridge when this happens, the unit on the bridge is destroyed with the bridge. Fire Dragons and Rock Elementals are immune to flaming arrows. If the Ice Drake is attacked with flaming arrows, the first sword on shield (SoS) result requires a critical hit check (not the first two hits, as with normal weapons). If a unit in a woods hex is successfully attacked (at least one kill…flag symbols do not count as a hit) by a unit with flaming arrows, the woods are set ablaze, and the unit in the hex must retreat (one hex, plus any additional hexes as required by flag symbols rolled). A unit that is Bold must also retreat from woods that have been set ablaze by fire arrows. Place a special marker on the woods hex. This hex is impassable for two turns (while the smoke clears...no units may enter the hex the turn the woods were set ablaze, and for two turns thereafter, meaning two turns of the player who set the woods ablaze). At the end of two turns, remove the woods hex from play. Units in water and swamp hexes cannot be affected by flaming arrows.