Part 3: Game Mechanics - Random EncountersGame Mechanics - Random Encounters
The Etrian Odyssey series employs a random encounter system to determine when your party will get into a battle. Preexisting encounters do exist, but these are either bosses or FOEs, and FOEs should generally be avoided when you first encounter them.
Each step taken in the Labyrinth increases the chance of your party being attacked. This is determined through several variables. Each step taken increases the Danger Level. When you enter a floor or get out of a battle, the Danger Level is set to 0. Tiles have Danger Values assoiciated with them, that increase the Danger Level by a certain amount. Walking over a tile adds that tile's Danger Value to the Danger Level. Some tiles in the Labyrinth have danger values of 0, meaning that you can never get into an encounter on those spots. The tiles near the entrance of the Labyrinth are an example of this. FOE infested areas tend to have tiles with low danger values, so it's unlikely to get into a battle as long as you made sure to empty out the Danger Level beforehand. There are abilites that can affect the danger value of the tiles, and the resulting numbers are rounded down.
Another value called the Trigger Level is set to a number between 16 and 64 inclusive each time you enter a floor, or get out of a battle. This means it could take 3 or 4 steps to get into a battle, or your party could explore a huge portion of a floor before finally fighting something. When the Danger Level exceeds the Trigger Level, the battle starts.
The game lets you know how close you are to getting into a battle with the indicator in the bottom right corner. Dark blue means its at 0%. It turns green when the Danger Level is 45% full. Dark orange by 60%, and dark red by 75%. It doesn't change color after that until you get into a battle.
The encounters you get into also depend on the tile you're on. So a monster that's a common encounter on one part of the floor might be a rare encounter on a different part.
Sometimes when getting into an encounter, you'll either have the chance to get a preemptive strike, or be blindsided by the monsters. The game determines this by using your party's and enemy's stats. There are exceptions to this and you can get into guaranteed preemptive strikes or get a guaranteed blindside by certain encounters. The formulas used to determine this use these values.
Party Rating = (Party's Highest AGI + 10) * (Party's Highest LUC + 10)
Enemy Rating = (Enemy's Highest AGI + 10) * (Enemy's Highest LUC + 10)
These two values are then used in this formula, which determines your party's chance of ambushing the enemy.
[5 * Party Rating / Enemy Rating]
The resulting number is rounded down. On average, fighting enemies of equal strength will give you a 5% chance to ambush them. The result is capped between 1 and 25 inclusive, meaning that the highest chance of getting a preemptive attack is 25%. Certain skills can affect the chance of getting a preemptive. If the preemptive strike fails, this formula is used instead.
[7 * Enemy Rating / Party Rating]
Again, the result is rounded down. Fighting enemies of equal strength gives the party a 7% chance to be blindsided. Again, the resulting number is capped between 1 and 25 inclusive, meaning that the blindside chance cannot go lower than 1%. Certain skills can also affect the blindside rate.
If the party doesn't get a preemptive strike, or get blindsided by an enemy, the battle starts as normal. It should be noted that some enemies will ignore these formulas and are always guaranteed blindsides should the party get into a fight with them.