Fire demon is long ranged unit which looks like lizard or wingless dragon.
It can delivery very long ranged fire ball attack which is as long as Stone Giant's attack. Note that these two units have the longest ranged attack among ground forces.
It is a tier 2 unit, and cost 1000 mana to build.
It can fire much farther than it can see an enemy on its own. Using other units to scout ahead, or aiming it by use of the radar, helps it greatly.
The unit doesn't move as fast as some other units.
It can not fire over objects, as a catapult would, but instead only in a straight line.
By selecting a group of them to control at once, you can effectively use them to target the same enemy unit at once, and kill things more effectively that way.