A trap is any device or magical ward that meets three criteria: it can inflict a sudden or unexpected result; the target would view the result as undesirable or harmful; and the creator specifically intended the harmful or undesirable result as such.
The chance to set the trap is equal to the Thief's skill value (in %), modified by luck. E.g. a Thief with 70 points in the Set Snare ability and a current luck of 1 will have a 71% chance to succeed whenever they try to set a trap.
The trap activates when an enemy gets within a certain distance (doesn't have to walk directly over it), and shoots out projectiles that hit all enemies in the area. Allies of the thief are not affected. The Thief can per rest set one trap at level 1, plus one for every 5th level thereafter, but also limited by the trap limit per area.
Thief ability traps damage:
- 1st level: Deals 2d8+5 missile damage
- 11th level: Deals 2d8+5 missile damage and additionally deals 2d6 Poison damage per round for the next 3 rounds
- 16th level: Deals 3d8+5 missile damage and 4d8+2 fire damage
- 21st level: Deals 3d8+5 missile damage and 20 Poison damage with no save, Slays target if a Save vs. Death with a +4 bonus is failed (and, being a nonmagical effect, it bypasses magic resistance entirely).
In addition, the kit Bounty Hunter for the Thief class gets access to the ability to Set Special Snare.
If chosen, some high-level class abilities allows one to get three types of traps; the Exploding, Spike and Time Trap.
Certain classes with enough skill points spend in the class ability Find Traps enables the detection and disarming of traps laid by enemies. The priest spell by the same name divines their location, but doesn't disarm them.
Setting a trap activates the same cooldown as casting a spell or using any magical item, meaning you can't use a potion or spell or set another trap within 6 seconds (1 round).
The trap limit in BG2EE is 6 per area. Modders can change this (or functionally remove it) by editing TRAPLIMIT.2DA.