The games have a number of captains, and they will sometimes fight among themselves. The winner will get stronger and level up. The player can attack these captains, too, who may have a strong point or a weakness. If the player dies while fighting, the captain grows stronger and will taunt the player on a rematch. If the player manages to make the captain flee and live, he may develop a fear for the player
There are many things that can define the captains, such as fears (of the player, of fire, of wargs), weaknesses (to stealth, to arrows, to fire) and even immunities (to arrows, to stealth, you get the drill)
Also, if the player dies to an unamed orc, said orc will become a captain. I vaguely remember some captains coming back to life, if they weren't decapitated on the killing blow. They return weaker, but will comment on wanting revenge.
As a final note, Shadows of Mordor is a pretty fun game. Think of it as Assassin's Creed exploration with Batman Arkham City combat. Not the most faithful game to Tolkien's work, but it's good.