The Defense of Arx Naboth by Jonoth
After the PCs side with Vodallus (the leader of the human rebellion), one of his commanders comes in to warn him that an entire Eladrin airship fleet has been spotted, and is coming to bombard the fortress (Arx Naboth) in which the PCs currently reside. Vodallus hastily pleads for the party members to aid him; he gives each a handful of glowing purple crystal shards. He then tells them to head for twelve ballista placements along the inner wall placing the shards on the bolts (anywhere on the bolts will do and the shards will automatically stick). He explains (if asked) that the purple crystals destabilize the magic used to keep the airships afloat, and that they are very rare and powerful. Vodallus was loath to use them unless the Eladrin felt it necessary to bring the full force of their navy to decimate Arx Naboth (which they have done).