Said circumstances include haunted King of the Hill matches, during which players must battle the powerful ghost/wizard Merasmus and his book of bomb magic, the Bombinomicon. Additionally, control points have been booby trapped and now trigger the "Wheel of Fate" with each change in power, which has as much potential to harm as it does to help.
Item-wise, the ghoulish update adds magic spells in the form of potion bottles that can be applied to other inventory items; changing their colors, summoning ghosts or performing any number of other possible magical happenings. "One of the spells might even give you Glenn Danzig's eldritch phone number," Valve's announcement reads.
Mann vs Machine isn't left out of the fun either, gaining a brand-new mission called "Wave 666," which replaces the usual robotic enemies with those of an undead variety.