Roy's Prophecy (#331)
That actually covers 4 of 6 prophecies now (and the 5th is most of the way there).
1) Haley - "How can I get my speech back?" "Don't look a gift horse in the mouth." -
led to date with Nale and Haley revealing her love for Elan (#393).
2) Belkar - "Do I cause the death of one of the following: Miko, Miko's stupid horse, Roy, Vaarsuvius, or you?" "Yes" -
Belkar killed the oracle (#567).
3) Vaarsuvius - "How can I achieve ultimate arcane power?" "By saying the right four words to the right person at the right time for the wrong reasons." -
V says "I...I must succeed" to herself to save everyone for the wrong reasons (#634).
4) Roy - "Will Xykon be within 1000 yards of Girard's gate or Kraagor's gate first?" "Of those two locations, Xykon will be within 1000 feet of Girard's gate first." -
Well, he was within 1000 yards (#899).
The other two:
5) Durkon - "When will I be able to return to my dwarven homeland?" "Posthumously." - So we know Durkula will make an appearance near Kraagor's gate.
6) Elan - "Will this story have a happy ending?" "Yes, for you at least." - Rich's reassurance that this is not a tragic story.