Veteran/Champion missions will start spawning the moment you get a lvl 3/5 hero, and it can affect where you would want to go if say a Long Champion mission is the only thing to appear in the cove when you're trying to kill the veteran Siren. One of the hardest juggling acts in Darkest Dungeon is managing your hero level in order to manage the mission difficulty pool.

Each difficulty indicates various things for the mission: The difficulty rank will determine how much progress you've made to the next boss, how much EXP your heroes will get, whether or not certain heroes will get stressed by going there or even bother going on the mission, and most importantly influence the enemy spawns of each area. These missions are color coded, with Green being Apprentice, Yellow being Veteran, and Red being Champion dungeons. The game only has four 'biomes,' however each one will have a different degree of mission difficulty depending on the level distribution of all your heroes.

One of the main things to note in Darkest Dungeon is the game's progression system. Part 9: Difficulties & Champion Monsters Difficulty & 'Champion' Enemies

Darkest dungeon cove champion