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

The base surprise chance is 10% for both sides. That is, without any modifiers (including light modifiers), there is a 10% chance of the party being surprised, a 10% chance that the monsters are being surprised and 80% chance for no surprise happening. Note however that in every light level someone gets bonus to their surprise roll.

The maximum surprise chance is 65% for both sides.

NOTE: While in the Darkest Dungeon (location) 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 few turns to get them back into optimal positions, prolonging the fight and taking extra damage. Increase your chance to surprise enemy with: Reduce negative impact of being surprised by:
 * trinkets
 * camping skills
 * keeping your light meter high
 * scouting battles
 * equipping moving skills (e.g. Grave Robber's Lunge, Jester's Finale)
 * equipping skills that can be used from multiple positions
 * using sub-optimal set of skills to end fight as soon as possible
 * increasing scout chance because heroes don't get surprised in scouted battles.

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