From "Star Wars: The Clone Wars: Witches of the Mist (#3.14)"
There were no survivors on Devaron. Everyone was killed, including Master Halsey and his padawan.
in the past, only Dooku's assassin was capable of such an effort. But she was presumed dead at the battle of Sullust.
Not everything is what it seems. There may be a new threat. Someone or something has eluded us, perhaps a newly appointed Sith Lord.
This is not the work of a Sith Lord or a Jedi, but a reckless, impulsive animal.