The Inferno is a description about the Inferno's architecture and units. It was written by employees at New World Computing and originally published on The Nether Gods, a Heroes III fan site that has since disappeared.
The story is copied in its entirety below.
The Inferno Edit
"My friends, in all my studies of Erathia, there is but one thing that fills me with dread more than the silent, cold death of the Necropolis, and that is the twisted, black-hearted evil of the Infernos. Foul creatures, ripped from the depths of the Hells and beyond, are bound into the service of the Demoniaks and Heretics who call them forth. They serve unquestioningly, and without fear, but are always looking for the one chance to break free from their master. Fortunately, I have never actually seen one of these creatures loose.
None of the creatures of the Inferno cities are native to this land. Because of this, there are few actual dwellings to be found. Instead, a great number of altars, halls, towers and pyres need to be erected in order to summon the Inferno's armies.
The lowliest creature to be found here is the Imp. Honestly, they are like rats, and no matter how many times you think you have killed them all, more seem to appear. Through research I was able to determine that once an Imp becomes a Familiar, it can create a dimensional rift within the city, allowing an even greater influx of these vermin.
After the Imps come the summoning halls of the Gogs and the furnaces of the Hell Hounds. Both appear in great numbers, though fortunately nothing near the influx of the Imps and Familiars. The Hell Hounds, when properly summoned, can not only take the form of a stronger Cerberus, but in the best cases can also be summoned in greater numbers.
As many of you have asked, there are in fact Demons to be found in Erathia. They, along with the weaker Hell Hounds and the stronger Pit Fiends, seem to make up the bulk of many armies of the Infernos. Strong alone, and deadly in great numbers, these creatures make even the holiest and hardiest of souls tremble.
Yet there are two creatures that outclass those that I have already mentioned. The first, summoned from great towers, are the Efreet. These monsters bear a striking similarity to the Genies, riding about on great waves of fire that always surround them. However, even the Efreet Sultans quiver with fear in the presence of the purest of all evil creatures, the Devils. Though they pledge their service willingly (for the right price, of course), it always seems like they come to this land for their own reasons.
Fear these creatures, friends, for they represent all that opposes Good. So ends today's lesson."