"Inferno towns can be found in Erathian regions blighted by the emergence of the underworld on the surface. Demoniac and Heretic heroes stand in uneasy alliance with these towns. Inferno armies have great hand-to-hand attack units and are only slightly hampered at the lower levels by a lack of flying units until the awesome, teleporting devil and arch devil units can be brought into play." RoE manual
Some may find the information in this section subjective or irrelevant.
Inferno armies have great hand-to-hand attack units and are only slightly hampered at the lower levels by a lack of flying units until the awesome, teleporting devils can be brought into play, as well as strong and fast efreet.
Castle Gate is a significant advantage of Inferno. It enables to only upgrade creature dwellings in one town, and use the gate to transfer them to be upgraded between towns. However, it's useless if the map has only one Inferno, where Castle Gate is possible.
Creating a Demon horde easily: Attack any creep on the map. Blind it, so the battle lasts as long as you want. Cast Fire Wall/Fireball/Inferno/Meteor Shower on your Imps. When they die, raise Demons, then finish the enemy.
Another good thing about the Castle Gate is that one hero can be used to defend all towns.
After the Lich, the Magog is the only one with attack in area but, unlike Lich, is compatible with Frenzy and Bless (the latter not present in Inferno).
The manual states that Inferno armies have great hand-to-hand attack units, and are slightly hampered at the lower levels by a lack of flying units until the devils arrive. The statement is supported by the fact that compared to their levels, many of the Inferno's creatures have relatively high speed values and maximum damage values. However, lack of flying units is untrue in the sense that Efreeti and Efreet sultans are definitely noteworthy adversaries. While Inferno troops are great for attacking they are not strong defenders which means that combat strategy with Inferno should be based on first strike.
Another weakness in Inferno's army is lack of shooters. Gogs and Magogs are the slowest creatures in Inferno's arsenal, which makes them a somewhat liability considering the mobility of heroes. Additionally, usefulness of Magogs' special ability to throw fireballs is controversial as they may inflict damage to allied troops as well (fixed in Horn of the Abyss as they can target any hex).
A good tactical idea is to use only Arch Devils and Efreeti, cast Anti-magic on Arch Devils and then Armageddon. It would also ruin your opponent's Dragogeddon tactics as Arch Devils are faster than Gold and Black Dragons.
Castle Gate: A life-saver on maps where you control more than one Inferno town.
Pit Lord's Demon summon ability: Lets you horde demons. Consider using Marius whose specialty is Demons as your main hero if using this strategy (she will increase Demons' speed as well). If using this strategy, there is almost no need to spend time and resources to upgrade Imps and Gogs or to even build the upgraded buildings since the upgrade does not increase their health (see Summon demons).
Efreet Sultans are very good at clearing a map: they have speed, damage, and enough health. They can also be used in a Dragogeddon combo. Consider splitting them into many stacks of 1 unit only in the early game without carrying any other creatures to reduce losses and to increase your hero's movement points.
This town has the highest chance to get the Blind spell in the Mage Guild.
Cerberus are very powerful with Bless/Haste/Counterstrike.
Both Devils/Arch Devils and Cerberi have no enemy retaliation. The hate between Angels and Devils gives a slight advantage to the latter, since Devils don't have retaliation, unlike Angels.
Moreover, hate between Genies and Efreeti is also useful to Inferno, because Efreeti are much stronger.
The -2 Luck decrease of enemies' Luck is a lot more useful in Horn of the Abyss than without the mod because it introduced negative Luck. Negative Luck gives every stack a chance of 1/12, 1/6, 1/4 with -1, -2, -3 luck to do about half damage only. Assuming the opponent has 7 stacks, on average 0.58/1.17/1.75 stacks will only do about half damage every round if the opponent has -1/-2/-3 luck. This is a significant advantage. Additionally equipped with artifacts that decrease the opponent's luck, this can prove to be devastating, especially against wandering creatures who do not have a hero increasing their luck through artifacts.
Familiars have 7 Speed. You can give them to secondary heroes to increase their movement points without affecting your primary hero's army strength. If you want, you could even hand out Cerberi for a speed of 8.
Cons
If all buildings can be built, Inferno costs the most gold to fully build up. (Castle Gate alone costs 10,000 gold and is not essential; Furthermore, Imps' and Gogs' upgraded buildings are not necessary when using the Demon Farming Strategy).
Lacking in the Hero department. (When using Demon Farming, Marius is not terrible at least, and Octavia gives gold when playing at Difficult/Impossible Difficulty).
Imps and Familiars are weak first level units, and have no redeeming features save their speed (the Familiars' Magic Channel ability is hardly noticeable). (But, they can be sacrificed to be raised as Demons).
The Inferno produces the least hit points per week of all the towns. (But it can Summon Demons which partially makes up for it. Also, it has a 20% chance of getting Resurrection in the Mage Guild).
Only one low-level ranged unit, and Magogs are ambiguous due to their opportunity to kill your own troops (which, on the other hand, may help in Demon farming).
No Bless spell in the Mage Guild, while Cerberi have a very big damage range (if they are cursed, they are very weak).