Surprise

Upon entering a battle, there is a chance to surprise enemies or be surprised by them. If your party is surprised, party members will be shuffled around, messing your formation. Escaping from battle becomes impossible for the first turn.

If you surprise enemies, they will go last on the first turn, allowing you to reliably plan actions.

There is a base 10% chance of being surprised, or surprising monsters, in rooms and corridors that haven't yet been scouted. But in corridors and rooms that have been scouted the base chance of surprising monsters increases to 25%, with no chance of being surprised. The chances differ again depending on the current light level, the specific increases can be seen in the below table:

The maximum surprise chance is 65% for both sides.

NOTE: While in the Darkest Dungeon, it is impossible to surprise your foes, rendering items that increase your parties' surprise chance obsolete.

Dealing with being surprised
Surprising your enemy is very good: you will have 4 turns before they get to act. Being surprised, on the other hand, can be extremely bad. E.g. having the Leper in the 3rd rank and a healer in the 1st; it will take a few turns to get them back into optimal positions, prolonging the fight and taking extra damage. Increase your chance to surprise the enemy with: Reduce the negative impact of being surprised by:
 * Trinkets
 * Camping skills
 * Keeping your light meter high
 * scouting battles (heroes don't get surprised in scouted battles)
 * Equipping moving skills (e.g. Grave Robber's Lunge, Jester's Finale)
 * Equipping skills that can be used from multiple positions
 * Using a sub-optimal set of skills to end the fight as soon as possible

Please note that Ambush, Shambler and Fanatic will always surprise you no matter what, so rearranging your Heroes into positions they don't want to be in before interacting with them may be a good idea.