Ekhidnae are a larger, stronger, breed of sirens appearing in The Witcher 3: Wild Hunt. They make bodies of water a dangerous place thus a crossbow might prove useful to Geralt.
The ekhidna is a legendary monster and a stronger variation of the siren. It looks more human-like, similarly to how the sea witch is depicted in the books. It attacks just like the siren, using its sharp claws. Ekhidna dwells in wetlands and is more active during rain. It is vurnerable to silver, fire and kinetic damage.
In Greek mythology, Echidna (from the ancient Greek: Ἔχιδνα, "She-Viper") was a half-woman, half-snake creature, and the mother of most of the monsters of the mythology.