Demons are a special unit in Stronghold Legends, and only available to players who are currently playing as the Evil faction.
They cost 5 gold, and 4 Honour and are recruited from the Sorceror's Tower. You can find out ways to generate Honour in the game here.
Demons are fast moving units who are relatively weak in combat, and their only real strength lies in the fact they leave a trail of fire behind them, to which they are immune from any damage.
They are also immune from any fire damage in the game by any unit, including Dragons and the Sorceror, but are not immune from damage by the Sorceror's special power Meteor Storm.
In fact it's far more likely that the fire left behind by Demons will cause more damage to enemy units than the Demons will do themselves.
They can also be killed by freezing by Ice units as well, which means that while the Green Dragon and Black Dragon both have no effect on them- despite the fact the attack command can technically be given to attack Demons and the Dragons will try to attack them- the Ice Dragon can freeze Demons and kill them that way. Giant Demonic Bats can also damage Demons or kill them entirely in their attacks.
The trail of fire left behind Demons will damage any other unit caught in its trail, including your own and any nearby aerial units, until the fire subsides. If you are planning to use these units, you should keep them well away from any other units in your army so they do not cause damage, especially in the case of armoured units.
In most cases, the Demons will only last a couple of seconds in combat, particularly if there is a group of enemy units they are trying to attack. Archers and other ranged units can pick them off within a couple of shots at the most. The trail of fire left behind also cannot damage buildings. If you would like a Demon to attack a building, you must specifically order them to attack it.
Demons are cheap units that are ineffective in combat, and very easily killed. They are however excellent units to use with harrassment tactics though. And especially if enemy troops are moving to a location in the middle of the map, these units are brilliant in getting into their formation and causing fire damage while they are moving.
Another excellent feature with these units is that they are one of the very select few units in the game that can actually climb up walls all on their own without the need for any siege equipment. This makes them incredibly useful for ambush or hit and run tactics while attacking an enemy castle, or, if you find a weak spot in an enemy castle defence you wish to exploit.
Tip: Note that Demons only leave a trail of fire behnd them while they're actually moving. If they are stationery, there will be no fire. Use this to your advantage when organising your army, or, attacking an enemy.
In this instance, you can bring White Witches or other aerial units up close to them and start killing them with no additional damage to yourself unless they move. Expect little of Demons in actual combat, treat them as cannon fodder to harrass and frighten the enemy, and you won't be disappointed.