Perhaps the inherent goodness of your character impresses the creature. It does not attack you, in any case. Rather, the tension in the air around you suddenly dissipates. The creature has gone, as if it were never there.
The two of you move forward. The only trace of the creature's presence you can find is a gray shroud, hanging from a tree. Not quite the encounter you'd expected, but you'll chalk it up as a partial victory. The Sihuanaba has departed, for the time being at least.
You wait until morning to relay this information to Claudette Byron, who tells you that this shroud is potently magical; handing it over to your research department will certainly boost the team's magical proficiency, if that's the direction you wish to steer their work.