In season 3, episode 9 of Teen Wolf, titled “The Girl Who Knew Too Much,” we found out that Jennifer Blake, played by Haley Webb, was indeed the Darach and the one who has been ritually sacrificing Beacon Hills citizens all around town.

Does Darach die?

She is killed by Peter Hale as she struggles to reach the Nemeton and her last hope of survival.

Who kills Deucalion?

Deucalion was ultimately killed by Monroe's Hunters, and his final words to were to Scott in an effort to remind him that Gerard knew that he couldn't beat Scott and his pack, giving him the reassurance he needed to defeat both threats with the help of his packmates.