In the games you'd have normal enemies that scale according to progress in the game. Like say Assassins creed where you start with basic goons with swords and later get armored crusaders to fight with.

But then you also would get a special enemy - mini boss orc that is a leader of a group/location or just a loner hunter. They will spawn at random in cities, and outside. Not sure if they can spawn at the missions tho. If you kill one, his death will be passed on his peers who will seek vengeance. Some of them will try to escape if they feel you are winning. They can creep on you again later. Sometimes orc can be resurrected and become stronger. There are also different types of them. Say one is bruiser, the other is tracker, third might be hunter type, and so on.

There are multiple difficulty tiers of these special mini bosses that are interconnected. Some have beef with each other and will infight. Some will team-up to kill you. Later in game there is a technique change that I will not spoil cause it is integral to the plot.

It is an extremely unique game mechanic that pretty much made Shadow of Mordor/War games into cult classics. It has multiple layers to it and makes each playthrough is quite a unique one. I strongly recommend to play at the very least the first one.

Kill a mini boss of an orc horde. Orc horde gains a new boss that evolves around how you killed thier predecessor. Rinse and repeat and the game gets harder and harder while you also git gud.