She mentioned that the creature was akin to those depicted in the works of a philosopher from her world, and that in those works, the creatures were capable of driving people to madness.

[She doesn't like dealing with hypotheticals like this, but she does come from a world where magic is real and mages are hired to defeat monsters attacking towns on occasion. It's at least vaguely possible... She twirls a lock of her hair around one finger idly in thought.]

If we accept the possibility that this creature has that ability, then perhaps the damage was inflicted internally by the crew, courtesy of hallucinations or paranoia.