At this point, roughly 98% of D&D SRD monsters are on the good side in my setting, I mean, they're too cool. So, I decided to throw something less cool, scary, intimidating and evil that's preferably not a nun.
Siren Head is a... well, this:
Siren Head started out as a scary picture with a short story attached to it. Now it became more fleshed out, even if that only means the speakers now have teeth.
Wanting to add something to make it more concrete, I made this addendum:
Siren Head is able to move those loudspeaker (looking) things around in 180° in both directions and aim them. The speakers are also capable of unleashing a sound-based attack that normally has the same effect on people as Justin Bieber's Baby, but it also has the potential to kill people.
Since I'm no Kurogiri, I can't make him open a portal to the 1883 Eruption of Krakatoa. It somehow has to produce a sound that can kill a person on its own.
That was until I discovered different damage mechanisms. Sonar can cause tissue shearing and hypothermia. Then there are also the directional speakers.
Though being sparing with energy is good thing, use as much as you need. Beware of what the loudspeakers can actually handle. Siren Head can't go deaf, it uses IR and sign language.
How could Siren Head instantaneously (or at least) kill a human, only with sound, in the most efficient way possible.
Note: There seem to be a few ways to kill stuff with sound:
Some info on Directional Sound: