In chapter 15, when Cervas starts summoning from the purple circles, some of the enemies come out invisible and intangible. I wasn't able to attack them but they could attack me. When I killed Cervas for the first time, she respawned invisible as well, and the night shifted back into day. I managed to finish the level by having an infected bear kill Cervas the second time. I'm playing on easy with the cheat weapon.