Demons are powerful creatures that can't be killed by ordinary means. They have only one goal : resurrect the Demon King, which is possible by creating three fruits of life, id est, by killing at least 150 000 people.
Actually, each time a hero spawns, 8 demons spawn as well. They are always the same every time except one whose power and name change automatically with each cycle.
There are two types of demons : Army-type demons and singular-type demons.
There is no doubt that both types are significant threats.
Army-type demon are the kind of demons that, like the orc demon who attacked the academy, can mass-produce underlings and have them crush everything as a mob. An army of over a thousand of them is troublesome, but conversely, those demon don't pack a punch. For Lugh, even before he received his new powers thanks to Epona, fighting army-type demons would’ve been no problem, but lone-type demons would’ve been difficult to deal with.
Singular-type demons can't create monsters, but they are extremely strong. We have three of them who already appeared in the LN : The Beetle demon, Gurt, killed in volume 3 by Lugh and his team, id est, Dia and Tarte with their new spell : Demon Killer ; Meena the snake Demon and The Beast King Riogel who isn't the type of demon that can spawn monsters, but he always has cronies around him, a harem of several females monsters.