The only resources you may play against automatic-attacks are ones that cancel the attack, cancel a strike, or would be otherwise playable during the strike sequence.

Helm of Her Secrecy
If Eowyn is in your hand, this card is playable on a company facing an attack (before strikes are assigned) -- the company must contain a character with Edoras as a home site. If enough influence is available to control her, Eowyn may be played with (i.e., joins) the company. She gains +2 prowess, +1 body, and +1 direct influence. If the attack is a Nazgul, place Helm of Her Secrecy with Eowyn following the attack. Otherwise, discard this card following the attack. Regardless, Eowyn remains in play.

Adunaphel Unleashed
Playable on any attack against Adunaphel the Ringwraith (as your Ringwraith) if she is the only character in her company. The number of strikes of the attack is reduced to one and the attackÕs body is modified by -2. Alternatively, playable on any attack by a lone Adžnaphel the Ringwraith (as your Ringwraith). You choose defending characters. Any resulting body checks for defending characters are modified by +2. Cannot be duplicated on a given attack.

Motionless Among the Slain
Playable on an attack. You assign all strikes of the attack regardless of the attack's normal capabilities and character status. +1 to all body checks by your characters resulting from the attack.

Sojourn in Shadows
Magic. Shadow-magic. Playable before strikes are assigned on a character facing an attack in a shadow-magic using character's company. Target character cannot be assigned a strike from the attack. Unless he is a Ringwraith, the shadow-magic using character makes a corruption check modified by -4.

SATED BEAST
Target ahunt Dragon manifestation is discarded. Alternatively, if Doors of Night is in play, decreases the number of strikes from one Dragon or Drake attack by one (to a minimum of one). This card may also be played during opponent's movement/hazard phase.

SEIZE PRISONERS
Playable during the site phase at an already tapped Free-hold. The company faces an attack (attack type as first automatic-attack at site-3 strikes with 8 prowess). If no characters are untapped after the attack, discard Seize Prisoners. Otherwise, tap 1 character in the company and put Seize Prisoners under his control. No marshalling points are received and that character may not untap until Seize Prisoners is stored at a Darkhaven, Shadow-hold, or Dark-hold during his organization phase.
Cannot be duplicated at a given site.

MOTIONLESS AMONG THE SLAIN
Playable on an attack. You assign all strikes of the attack regardless of the
attack's normal capabilities and character status. +1 to all body checks by your characters resulting from the attack.

[...]As an exception, if one of the effects of a card is an attack, cards may be played that cancel the attack, cancel one of the strikes, or that otherwise are playable during the strike sequence.