Great use of the theme. It's a great concept, evil lurking just outside the lander. You've given the Siren some great hiding options to help the encounter from resolving too early. I do wish there was a little more Warden-facing strategy, some hints on when to use which abilities. Since the Siren Song only triggers on a failed Panic Check, it might be nice if there were a few more places for those to happen in the module as well.
Otherwise, great stuff. You've really nailed the heart of horror, I think: a doorway and something mysterious and frightening on the other side. Well done.