Council of Elrond

Playable on a site with an Orc or Troll automatic-attack. When any item is played at this site, the company must face all automatic-attacks of the site again with the attack's prowess modified by +3. Any successful strike does not harm the character, but rather the character is taken prisoner at the site. The rescue-attack equals all automatic-attacks of the site at the time of rescue.

Scenario: Forewarned Is Forearmed is in play. Company at hero Iron-Deeps has defeated isolated AA [Trolls (1st attack) -3 strikes with 9 prowess] and Forewarned Is Forearmed is discarded. Now company is playing item and Troll-purse played on-guard is revealed.

Question is:
If 2nd AA was not defined when company was facing AA first time, can it be defined now by hazard player (when Troll-purse is triggered)?

you would face all the auto-attacks. Note though, that if you hadn't had Forewarned in play and faced the 2nd AA at the beginning of the site phase that attack isn't strictly duplicated by Troll Purse. Your opponent has to play another creature with the appropriate playability requirements for the 2nd AA.