The Dark Tongue is the arcane language of the Forces of Chaos. A ritual language, its phoentic runes are the only manner in which the mysteries of Chaos can truly be expressed. It is the native language of daemons and Chaos creatures with the power of speech. The Chaos Space Marines of the Thousand Sons Traitor Legion in particular strive to learn this arcane language for the conjuration of Chaos entities and to bargain with daemons when they are summoned.
- War Zone Fenris - Wrath of Magnus (7th Edition), pg. 131