Echthroi (Ἐχθροί) is a Greek plural meaning "The Enemy" (literally "enemies"). The singular form of the word, Echthros (Ἐχθρός), is used in many versions and translations of the Bible for "enemy".
Historically used primarily in connection with biblical and classical subjects, the term has more recently been used to refer to a fictitious type of evil being, principally in Madeleine L'Engle's "Time Quartet". A personification of the forces of impersonalization and nihilism, they exist in both the macrocosmic and microcosmic level, counteracted principally by what L'Engle refers to as "Naming", or re-integration of a character with its best-motivated identity ('true self'). These concepts appear in one form or another in a number of L'Engle's books, as part of her recurring themes of good versus evil, interdependency, and the role of the individual in the cosmic scheme of things.
L'Engle's Echthroi first appear as antagonists of A Wind in the Door (1973, ), wherein Meg Murry first becomes aware of their existence when a doppelganger of school official 'Mr. Jenkins' suddenly flies "screaming across the sky," becoming "a slash of nothingness". The "singular cherubim" Proginoskes later identifies this doppelgänger as one of the Echthroi, and demonstrates their specialty by showing the Echthroi annihilating stars. Meg's father, an astrophysicist, has already been called to investigate these phenomena, while Meg's mother, a microbiologist, is simultaneously investigating an illness in Meg's brother Charles Wallace Murry's ;–– both caused by the Echthroi. Later the Echthroi again impersonate Mr. Jenkins, and convince the microscopic "farandolae" to behave destructively toward their host , nearly killing Charles Wallace, only to be overcome by Meg's 'Naming'.
Charles Wallace again runs afoul of the Echthroi in A Swiftly Tilting Planet (1978, ), wherein he and the unicorn Gaudior contend with them for control of a certain ancestry, which produces either villains or peace-makers according to the result of this competition.