Red Dragon and Black Dragon
• Breath attack
• Immune to spells level 1–3
• Breath attack
• Immune to all spells
• Hates Titans
"Red and black dragons with their great mobility, flying ability, and powerful breath weapons are among the most effective units in the game. Red dragons are immune to spells below level four. Black dragons are twice as durable, do 150% damage to... titans, and are immune to all spells." RoE manual
This spell immunity is a double-edged sword, though, as it makes both Red and Black dragons immune to most beneficial spells, including Resurrection. Placing Red or Black Dragons (as well as any living Dragons) in the Skeleton Transformer will give you Bone Dragons instead of regular Skeletons.
Both dragons are affected by Dispel.
Though it is stated in the manual, Black Dragons do not hate Giants.
Tactics and info
With the health value of 300, Black Dragon is one of the toughest creatures in the game. In the original Restoration of Erathia, it was the toughest of the dragons and its health was only matched by Titans' and Ancient Behemoths' (and by Haspid's in Horn of the Abyss). It also has more defense than either of those creatures, but not necessarily by much. Armageddon's Blade expansion introduced four new dragons, Faerie Dragon, Rust Dragons, Crystal Dragons and Azure Dragons, which all are neutral creatures and have more health than Black Dragons.
While not having the highest damage of a level 7 creature, they still have it higher than most of them, accompanied by the Breath attack ability, which lets them strike two units at once. They also have a respectable attack value, but lower than the Gold Dragon's by 2. Black Dragons also deal 150% damage against Giants and Titans.
Efficiency and cost
To build the base dwelling of this creature, all other base creature dwellings are needed together with the Mage Guild level 2. However, unlike their nemesis, the Giants and Titans, the dragon's base dwelling requires much more valuable resources, which usually results in getting Red Dragons later than someone would get Giants, and also having less of them to upgrade in the upcoming weeks.
Because of the Black Dragons' spell immunity, they are suitable to be used in tactics often called dragogeddon. In dragogeddon, a hero with preferably high power skill and Armageddon spell attacks with an army only consisting of creatures immune or very resilient to the Armageddon spell. The tactics may be used in "hit-and-run" tactic, to advance through wandering creatures or even to slay enemy heroes. In Restoration of Erathia series, Black Dragons were the best creature to use in dragogeddon.
The benefit of using Black Dragons in this tactics is their high initiative, in other words, their speed. Only Phoenixes and Gold Dragons are faster creatures that are immune to Armageddon. High initiative enables hero to act first in the combat, and this way possibly win the battle before enemy gets a chance to flee, surrender or otherwise cause resistance.
Even so, the Black Dragons is technically an excellent fighter, being able to reach the enemy shooters the first turn, but they won't last too much against an army with similar strength, since they can't be normally resurrected, so they have to be well backed up by other friendly troops.
The main tactic against Red and Black Dragons is to simply attack them with all you've got and overwhelm them. Without the dragons, a Dungeon army is not that much of a threat (with the exception of Minotaurs and/or creatures from banks), unless it has accumulated absurd numbers of creatures, or is led by a way more powerful hero than yours. Also, be careful with your troop placement, as to not let them strike two of your troops at the same time.
While not getting such an immense bonus from skills, like creatures as Titans do, there are still some good skills to take in consideration. Armorer is a very good to choice to keep the dragons' numbers up, while Tactics allows more friendly troops to assist the dragons easily, which can be a big advantage for a Dungeon army to deliver a first, strong strike. Offense is, as usual, a nice skill to have, especially with the dragons' Breath attack and the Minotaur and Minotaur King's large damage range.
Together with the Titan, the Black Dragon has become a symbol of the Heroes series since it first appeared in Heroes of Might and Magic II: The Succession Wars.
In one-on-one combat with no stats bonus, a Black Dragon barely defeats a single Titan. However, a difference of 1 more attack or defense for the Titan makes them win instead.
Comparing the Black Dragons in the in-game cutscenes, fans estimate they are ~24 metres tall.