Sirens are a water beast featured in the Spook's Sacrifice.
Water creatures from that inhabit the coasts of Greece. While lounging at the rock's near the waters' edge they lure sailors to their deaths by using the "sirens song".
This song places sailors under a hypnotic spell that makes them think that the sirens are beautiful women when in reality they are hideous with huge fangs and swollen lips.
The enchanted sailors try to reach the sirens and die horrible deaths trying to do so. The sirens feed upon the mens' flesh.
Sirens are usually found in the ocean, and this means that their curse only affects sailers, or those who go out to sea. They are commonly found in Greece.
Effects on Seventh Sons
Spook's have some immunity to the sirens spell which gives them a better chance at survival. If you're not a seventh son of a seventh son don't loose hope because there is a way to evade the power of the sirens, it being to press wax into your ears so that you don't hear their song.
If the "sirens song" doesn't reach your ears then it can't affect you and you can sail on to safety.