A Demon Army, is an organised big-scale force of Monsters led by a Dark Lord, usually to attack or invade a human territory. Their numbers tend to be massive to be called an Army, but they obviously vary wildly depending on the scale of the invasion.
Nowadays, they are not seen so much in human countries due to the inner conflicts the Monster Realm is facing in the current era, but both factions of the Monster Realm, the Kayblis Faction and the Hornet Faction, have several Demon Armies and are constantly fighting against each other.
The only two examples where shown in Rance VI, with Dark Lord Kamilla's invasion over the Kingdom of Zeth, and in Sengoku Rance, when Dark Lord Xavier formed an army to destroy JAPAN upon being unsealed.
At the top of a Demon Army, there's the Dark Lords. In the case that there's more than one, they all have the same military power unless they agree otherwise, or if one of them is part of Elite Dark Lords.
They represent the biggest threats and are the ones that formed the Army to begin with, usually for personal reasons. Taking them down is the equivalent of defeating the Demon Army instantly, but Dark Lords are extremely powerful creatures and can only be defeated by going through their invincibility field. Sealing them is also an option but this is usually a very hard task since most Dark Lords are strong enough to render most seals worthless unless they are very weakened.
Apostles are the direct servants of the Dark Lords. They are also very powerful and lead monsters just the same, usually directly in the battlefield.
They are the Dark Lords most powerful servants and their status is above any monster. They don't have the invincibility field, but most of them are stronger than any human in a 1v1 scenario. They don't age but they can be killed.
Monster Shoguns are incredibly intelligent monsters that can have up to 20,000 monsters under their command in the battlefield. They are powerful, tactical and charismatic leaders. They can come up with underheaded and smart tactics comparable to that of a skilled human tactician.
They capture females and keep them within their inner boddies using their minds to improve their own intellects. They tend to be loyal but they are still monsters and can sometimes act base on insticts as well.
A very humanoid type of monster, with blue and muscular bodies. They wield big and heavy swords, and are primarly physical figthers.
They are also smarter than the average monster and can lead battalions of monsters with great profficiency. However, their leading is not much more complex than charging and killing the enemy. They can lead up to 200 monsters.
Red Soldier Monster
Soldier Monsters are humanoid in form and they are pretty strong as well.
The Red Soldiers are the most powerful of the class, being three times stronger than the green class, and fight wielding an axe.
Blue Soldier Monster
The Blue type are the middle-class of Soldier Monsters. They use something that resembles a worm as a whip.
They are not much stronger than the Green type, but they have a longer range than other Soldier Monsters and can hit back-units.
Green Soldier Monster
The weakest type of Soldier Monster. Like the red ones, they fight wielding an axe, but not nearly as strong.
They are decently strong and have a pretty good defense, but they don't have much else going for them. They mostly rely in overpowering their enemy with numbers.
Aside from all the mentioned above, lots of different types of Guy Monsters and even Holy Skeletal Fighting Marshal are also part of the Demon Army. There are lots of monsters of different shapes and sizes, like Akames, Dekants, Bambaras, Chasobas and even Okayu Fever.
Sometimes they can have higher positions depending on their individual performances. They are not necessarily lower ranked than soldier monsters.