RTF is a global modification that replaces all towns in the game.

Are demons degrading? For me, a demon is a somewhat intelligent creature, while a giant worm is just an animal (that chases after a Saret faster than it can think).

Well, what else can you expect from Chaos? It's change, mutation, adaptation... and intelligence isn't always necessary for survival.

Dogs and worms are servants of Gluttony; they become dumber, but in reality, it's not just them that are degrading. The demons of Reproduction are also degrading – they can become Progenitors, like in Heroes 6, but there are also "stationary" Progenitors that grow into the ground and are building-sized. By the way, in Heroes 5, it is mentioned that Imps are caught in lava lakes during low tide; perhaps the Progenitors live at the bottom.

In principle, the Madmen and the Demons of Destruction are also degrading, but I think there are always exceptions. For example, Deleb seems to belong to the servants of the Lord of Pain – she creates torture devices, but she is a Succubus, so she cannot become a sadist.

Nibiros is another interesting character; he guards the Progenitors and is considered a servant of the Lord of Reproduction.

Everything is ambiguous with the Lord of Madness; he is definitely not stupid, and his army includes huge, insane giants who can compete with Sadists and Destroyers.

In Heroes 5, it is mentioned that there were 8 Lord-Progenitors, and in Heroes 6, there are 6. This is, of course, a retcon, but you can build theories around it, such as the existence of the Lord of Horror – the master of nightmares – and another unnamed one. Perhaps they were destroyed before the fall of Shantipri, or during the fall; who knows.

And after the reign of the Overlord, anything could have happened