This hurts Gravedust in ways he’ll find difficult to express. His life’s purpose is to bring peace to lost souls. It’s not as if he’s never failed in any way before; there were plenty of people at B’ial Vezk his empowered arrows weren’t fast enough to save.  Perhaps now and again he failed to help a soul find peace, in which case it would just keep wandering (example: Ashok). But never in his life has he taken an action that led to the destruction of one of the souls in his charge.

This could be seen as a stumble that will ultimately lead him to a new path. But right now, for him, it just feels like an ending. He sinks to his knees, paralyzed with regret.

Perhaps, on some level, the creature senses this paralysis as it lashes back against what threatened it.