rakshasa
Rakshasas are an unusual breed of creature, which deals deep within the darkness of the world. Though unrelated, they get along relatively well with illithids, and you may stumble across them plotting together.
Rakshasas are powerful spellcasters, capable of many tricks, including invisibility and the ability to make illusory copies of themselves. They are masters of using arcane energy to harm their foes, and prefer to fight at a distance.
Being spellcasters, rakshasas occasionally carry tomes of power with them. While not useful by themselves, they can be used to imbue powerful magic into weapons, armor, or tools.
Rakshasa as found in Dungeon Mobs are from Dungeons & Dragons, and have been described in the game manuals as "evil spirits clothed in flesh." They are mid-level enemies with prominent magical ability (stats here).
The original rakshasas come from Hinduism, where they are enemies of the Hindu gods. The word "rakshasa" is usually translated to English as "demon."