After escaping Fort Dragonfall, you seek a hidden passage from the Rainbow Sage's map.
Defeat the Boss
In this map, your walking units will not be able to go north until a royal unit uses a Dragon Vein to create a passage. However, on each level, there are two Dragon Veins, and one of them is the "wrong" Dragon Vein, which will turn the tiles into lava tiles, and one of them is the correct Dragon Vein, which will create the passage to the north. It's easy to tell which is which: the statue above the Dragon Veins is holding a crystal ball on the side with the correct Dragon Vein. Just use the Dragon Vein below the statue's crystal ball and you will be fine every time.
If you do use the wrong Dragon Vein, the lava tiles on the floor will slow your movement and damage you. However, there are holes in the ground that a unit can stand on to have an increased movement rate and also receive healing at the start of the turn.
Several enemies here will drop items when you defeat them, so be sure to look for those.
Some of the enemies here are Stoneborn, which are very strong and can throw rocks five tiles away, but they have low Res, so send your tome/scroll-wielding units to kill them.
At the top of the map, when you attack the boss unit, it will flood the top area with lava. As soon as you kill the boss unit, the battle will end.