D&D Monsters: Noncorporeal Undead

The concept of ghosts or other restless spirits is an ancient one, found, in some form or another in pretty much every human culture. The details vary considerably, and ghosts may not necessarily be malevolent, or even frightening but the basic idea is essentially universal. They're the sort of thing that just had to be included in D&D, albeit with the addition of generally making them more dangerous - and thus a meaningful threat - than they usually are in legend. There are several different types in the game, arguably a different sort of being from the corporeal undead, united by lacking a physical body and with various other means of inflicting harm. Four of these types are consistently present in the core rulebooks, although their details can vary significantly.

D&D Monsters: Succubi

The word "succubus" comes from Latin, referring to a type of demon that seduces men and slowly saps them of their vitality over a series of sexual encounters. This basic concept, however, long precedes even the Romans, with the general idea of  'evil demonic seductress' being fairly common across many cultures. It may, in part, be connected with the same sort of hypnogogic phenomenon that is responsible for the night hag myth - albeit interpreted in a more pleasurable manner. Indeed, succubi were not originally regarded as being attractive in their natural form - although, as with the D&D creature, they could shape-shift - the 'sexy' form only starting to become the standard in modern times.