Abdul Alhazred is a fictional character created by American horror writer H. P. Lovecraft. He is the so-called "Mad Arab" credited with authoring the imaginary book Kitab al-Azif (the Necronomicon), and as such an integral part of Cthulhu Mythos lore.
Abdul Alhazred is not a real Arabic name, and seems to contain the Arabic definite article morpheme al- twice in a row (anomalous in terms of Arabic grammar). The more proper Arabic form might be Abd-al-Hazred or Abdul Hazred. In Arabic translations, his name has appeared as Abdullah Alḥaẓred (عبدالله الحظرد): Arabic ḥaẓraحظر = "he fenced in", "he prohibited". Hazred could come from the Persian or Arabic word "Hazrat" meaning Great Lord with a twist that makes it sound like "red" and "hazard" both indicative of danger.
Similarly, an article (written from an in-universe perspective) in the Call of Cthulhu RPG speculates that it may be a corruption of Abd Al-Azrad, which it claims translates to The Worshipper of the Great Devourer.
The phrase "mad Arab", sometimes with both words capitalized in Lovecraft's stories, is used so commonly before Alhazred's name that it almost constitutes a title. A reference to the "Mad Arab" in Cthulhu Mythos fiction is invariably a synonym for Abdul Alhazred.
In 730, while still living in Damascus, Alhazred supposedly wrote a book of ultimate evil in Arabic, al-Azif, which would later become known as the Necronomicon. Those who have dealings with this book usually come to an unpleasant end, and Alhazred was no exception. Again according to Lovecraft's "History":
August Derleth later made alterations to the biography of Alhazred, such as redating his death to 731. Derleth also changed Alhazred's final fate, as described in his short story "The Keeper of the Key", first published in May 1951. In the story, Professor Laban Shrewsbury (a recurring Derleth character) and his assistant at the time, Nayland Colum, discover Alhazred's burial site.
While the two are heading a caravan from Salalah, Oman, they cross the border into Yemen and find the unexplored desert area that the Necronomicon calls "Roba el Ehaliyeh" or "Roba el Khaliyeh" — presumably a reference to the Empty Quarter or "Rub al Khali".
At the center of the area they discover the Nameless City (the setting of the Lovecraft story of the same name) and in Derleth's text the domain of the Great Old One Hastur. Shrewsbury, an old agent of Hastur and the devoted enemy of Hastur's half-brother, Cthulhu, crosses its gates in search of Alhazred's burial site.
He indeed finds Alhazred's burial chamber and learns of his fate. Alhazred had been kidnapped in Damascus and brought to the Nameless City, where he had earlier studied and learned some of the Necronomicon's lore. As punishment for betraying their secrets, Alhazred was tortured. Then they blinded him, severed his tongue and executed him.
Although the entrance to the chamber warns against disturbing him, Shrewsbury opens Alhazred's sarcophagus anyway, finding that only rugs, bones, and dust remain of Alhazred. However, the sarcophagus also contains Alhazred's personal, incomplete copy of the Necronomicon, written in the Arabic alphabet. Shrewsbury then uses necromancy to recall Alhazred's spirit and orders it to draw a map of the world as he knew it. After obtaining the map, which reveals the location of R'lyeh and other secret places, Shrewsbury finally lets Alhazred return to his eternal rest.