The OFFICIAL 'Help Me, Oh Jesus' Thread (Quick Questions/Clarifications)

[The Future Warrior]

When someone else makes a challenge. When the challenge is already made and the fight starts (assuming the party being attacked doesn't manage to escape, if they so choose to) can someone else jump in and crash the party, to help one side or the other? Or just bring chaos and attack both sides? Or is it set in stone once the challenge is issued?