Danger Sense

This proficiency provides a humanoid character with a sixth sense which warns of impending danger. On a successful check, the character avoids a trap at the last second or realizes that opponents wait to ambush him due to a sudden warning tingle that cannot be ignored. Characters who make successful checks spot traps before blundering into them and receive initiative against hidden opponents. This proficiency does not work against opponents who are out in the open and making no attempt to hide their actions. Failure indicates that the character senses nothing out of the ordinary and play continues normally.

Barbarians

This proficiency provides the character with a sixth sense that warns him of impending danger from traps, hazards, and ambushes. When the character is approximately one round away from triggering the threat (for example, in one round he'll step on a rattlesnake if he keeps moving at his current rate), the DM makes a secret proficiency check. If the check fails, the DM tells him nothing. If the check succeeds, the character feels a tingling in the back of his neck or on the tips of his fingers; the DM tells him the general direction of the threat (in front, to the right, overhead, and so on). The character doesn't learn anything about the type of threat; it might be pool of quicksand, a concealed lion, or a hidden trip wire. It's up to the character to figure out how to respond to the warning.

If the threat is an impending ambush from an NPC or a creature, the character automatically gains the initiative on the first round of combat (assuming that combat ensues one round after the character is altered by his danger sense). The DM may decide that the character's danger sense does not work against unfamiliar dangers. If an island barbarian has never seen a poisonous snake, he might be oblivious to the rattlesnake's danger. Exotic magical traps or illusions may also circumvent danger sense.

[Complete Book of Humanoids]
[Complete Barbarian's Handbook]