The Black Dragon is a very strong unit who can breathe fire onto the ground. In multiplayer and depending on the circumstances, the black dragon can do devastating damage
His firebreathing attack affects a large area. Like any other dragon in the game, it disappears after a certain amount of time, until death or when you disband it. The black dragon is the cheapest of all dragons, costing only 750 Gold, whereas green and ice dragons cost 1000. The reason the dragon costs less is because evil cannot use church services, nor bread, wine, etc. to gain as an extra popularity or honor. That's why an evil player needs to play offensively and as quickly as possible to convert, slay other units and capture new estates (and earn the needed ressources).
Make use of the dragon quickly, to soften up the enemy defenses first or chase down an enemy group. Pay attention to not lose too much time with it. A dragon is very valuable. Dragons are susceptible to crossbows but they can also counter them once attacking them first.
Try this OP strategy, banned in multiplayer: use suicide bats as the bats take out most troops, so the dragon will finish them off. Bats are, like the dragon hired from the Sorcerors Tower.
Dragons, when supported by other troops (especially werewolves due to their speed), can easily overtake stronger armies and defenses. Use the dragon to soften the enemies defenses or launch a counterattack, either way their power is formidable.