Can't kill Morvudd in the Missing Son quest in Witcher 3
This turns out that to kill Morvudd you can't be under influence of Cat potion.
Detailed information (has minor quest spoilers):
Morvudd is scripted to hypnotize you after his health drops under certain threshold, after that he runs to his lair. Cat potion (at least enhanced one) blocks hypnosis, so he can't run.