Shaken

Courage −0.15 and faster nerve loss. Never rolled at random: it comes from a hero's story. As leader, a Shaken hero calls retreat one step early.

In the fight

Shaken changes two of the values the combat AI builds for each hero:

Courage −0.15. Courage decides how long a hero keeps fighting as a fight turns bad. For an average hero (Bravery and Resolve at 10) it starts at 0.6, so Shaken takes away a quarter of it.
Steadiness +0.05. Steadiness sets how fast nerve drains once it starts going. A hero at Composure 10 goes from 0.35 to 0.40, about 14% faster.

Damage is untouched. The practical effect is a hero who falters earlier in hard fights, and who is more likely to drop a “press in” stance when hurt: below 60% health, the chance per check is 35% of how far courage sits below 0.8, which comes to about 12% for an average Shaken hero against 7% for an unshaken one.

How a hero becomes Shaken

Shaken is one of two traits the game never hands out when it generates a hero (the other is Child of the Halls). It comes from what happens to them:

The dark leaves a mark. After a delve, a hero who was wounded has a 10% chance to come home Shaken or Superstitious, picked at random between the two it does not already have. A hero who was not wounded but lost a comrade on that delve has a 4% chance. A hero who has already earned four traits over their career is spared.
The Cold Streak flaw. A hero whose personal flaw is Cold Streak behaves as if Shaken for as long as the flaw is part of their stakes.

The mirror image exists too: after a cleared delve with a delve rating of 85 or more, one wounded hero has a 15% chance to come home Brave or Iron-Willed instead.

As party leader

You set the order for when a party should retreat: after the first knock-down, at half, at most, or never. A leader’s temperament shifts it. Bold traits (Reckless, Dauntless, Brave, Glory Hunter, Restless, plus Hopeful) push it a step later and fearful ones (Cowardly, Shaken, Haunted, Brittle, Superstitious) a step earlier; the net sign decides. A Shaken leader with nothing bolder calls retreat one step earlier than you ordered. Steady leaders (Stoic, Cool-Headed, Methodical, Steady, Iron-Willed, Unbreakable, Stalwart) always follow your order exactly.

At a dungeon fork, a Shaken leader leans +2 toward the known path, taking a side room only about 20% of the time with nothing pulling the other way.

Elsewhere

Festivals. Shaken counts against a hero in wit (−3) and valour (−4).
The sealed tomb. A Shaken, Cowardly or Superstitious hero may refuse to break a tomb’s seal (“let the dead keep their gold”) unless an Iron-Willed, Brave, Professional, Stalwart or Methodical hero is there to overrule them.
Form. “Shaken” is also the name of a bad delve mark (−6 morale). The mark and the trait are separate things.
Flavour. On pair duty the hero speaks with the “nervy” voice. The hero card links the trait to Composure and Bravery; that link is descriptive.

Managing a Shaken hero

Keep them out of the leader’s seat on hard delves.
Pair them with steady heroes and a strong leader, whose aura adds to effective courage.
Watch their stress. A nervous hero in a hard fight builds stress quickly (see hero state).
Do not overreact. One delve’s scar is a moderate penalty; the hero’s class and attributes still matter more.

Combinations and history

Shaken has no traits it is barred from. On the same hero as Brittle, the courage penalties add up to −0.41, about as nervous as a hero can be. It is one of the original traits, present in the wiki’s earliest archived build, 1.9.6.

Recruiting

A moderate negative on any hero you plan to put in hard fights or in charge. Because it is earned rather than rolled, it is worth asking where it came from: a recruit who picked it up on a single bad delve is otherwise the same hero they were before.