Recently a bit of a contradiction has occurred to me. Namely that the actual source of the Legion’s fabled invincibility has seemed somewhat contradictory. Chronicle Vol. 1 told us that demons always return to the Twisting Nether to be reborn when destroyed. That’s why Sargeras had to create Mardum to imprison demons. Slaying them merely sent them to the Nether to regenerate.
However, it was eventually discovered that a death in the Nether was a true death. Illidan himself says so several times, it’s also said in Chronicle that a death in the Nether or in any place sufficiently suffused with fel energy would be a permanent death for a demon. Both Archimonde at the end of Warlords of Draenor and Kil’jaeden at the end of the Tomb of Sargeras raid are said to be slain in places of heavy fel power and thus presumed to be permanently dead.
This article is full of spoilers for Patch 7.3, Antorus the Burning Throne, and the end of the Legion expansion. If you read it, you will be exposed to those spoilers.