Not to be confused with the similarly named Alp-luachra, the Alp is sometimes likened to a vampire, but its behavior is more akin to that of the incubus. It is distinct from both of these creatures in that it wears a magic hat called a Tarnkappe, from which it draws its powers. The word Alp is the German form of the word that comes into English as elf, both descended from Common Germanic. It is also known by the following names: trud, mar, mart, mahr, schrat, and walrider. Many variations of the creature exist in surrounding European areas, such as the Drude or, in the British Isles, the Old Hag, related to now commonly recognized sleep paralysis.
In the period before about 1000, the Old High German word alp is attested only in a small number of glosses. It is defined by the Althochdeutsches Wörterbuch as a "nature-god or nature-demon, equated with the Fauns of Classical mythology ... regarded as eerie, ferocious beings ... As the mare he messes around with women". Accordingly, the German word Alpdruck (literally 'elf-oppression') means 'nightmare'. There is also evidence associating elves with illness, specifically epilepsy.
In a similar vein, elves are in Middle High German most often associated with deceiving or bewildering people "in a phrase that occurs so often it would appear to be proverbial: 'die elben/der alp trieget mich' (the elves/elf is/are deceiving me)". The same pattern holds in Early Modern German. This deception sometimes shows the seductive side apparent in English and Scandinavian material: most famously, the early thirteenth-century Heinrich von Morungen's fifth Minnesang begins "Von den elben virt entsehen vil manic man / Sô bin ich von grôzer lieber entsên" ("full many a man is bewitched by elves / thus I too am bewitched by great love"). Elbe was also used in this period to translate words for nymphs.
An Alp is typically male, while the mara and mart appear to be more feminine versions of the same creature. Its victims are often females, whom it attacks during the night, controlling their dreams and creating horrible nightmares (hence the German word Alptraum ["elf dream"], meaning a nightmare). An Alp attack is called an Alpdruck, or often Alpdrücke, which means "elf pressure". Alpdruck is when an Alp sits astride a sleeper's chest and becomes heavier until the crushing weight awakens the terrified and breathless dreamer. The victim awakes unable to move under the Alp's weight. This may have been an early explanation for sleep apnea and sleep paralysis, as well as night terrors. It may also include lucid dreams.
The Alp is best known for its shapeshifting abilities, similar to the creatures from werewolf lore. It may change into a cat, pig, dog, snake or a small white butterfly. It has also been said that it can fly like a bird and ride a horse. The Alp always wears a hat, giving it an almost comical appearance. The hat is known as a Tarnkappe (the literal translation being "camouflage cap" or "cap of concealment") which is simply a hat (or less commonly a veil) that gives the Alp magic powers and the ability to turn invisible while worn (see also cloak of invisibility). The hat is visible no matter what shape the Alp takes. An Alp who has lost this hat will offer a great reward for its safe return. The Alp also possesses an "evil eye" whose gaze will inflict illness and misfortune. Removing or damaging this eye also removes the Alp's malicious intentions.
It is not so terrible anymore in context with this post when you take into account that nightmare translates to German as "Albtraum" or "Alptraum", so a dream of an Alp. Furtherly, the painting in this post is called "Nachtmahr", which not only has the same meaning, but is very very close to the word "nightmare".
including a prayer to be said at night