As the adventurers head out individually into the streets of Synd, they can see shadow move among the flames atop it. Its brightness creates a large contrast to the blackness of the rest of the cave and a form coalesces -- large, hulking, and winged, it moves towards the ruined edge of the Spire and lets out another terrible cry. As it does so, a yellowish green fog starts seeping down the tower's sides near it. Within moments it has reached street level; Durrost and Alissa, being closer to the tower, can see all too quickly the effect it has as it moves in a rolling wave away from the tower and out into the city: everyone it touches falls to the ground and remains there, unmoving. Its progress is slow but inevitable as it moves down the streets.

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Durrost does not see anybody in the immediate vicinity who was injured by the explosion. The strange fog, on the other hand...

Those who are able may make spellcraft checks.
@Vassi
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You see a smaller humanoid form standing next to the larger winged one.