Burning Legion Missive
Source: Drop. Edit: The item has been updated and he now says new different lines. I will note which phrases are now removed with "old" and new phrases with "new".
The 
Burning Legion Missive is a toy that summons a hologram of Archimonde who speaks one of nine different lines, and it drops from Kharazos the Triumphant, Galzomar, and Sikthiss, Maiden of Slaughter in Talador.
Guide awaiting review
Travel to Talador
Head to Talador and look for Kharazos the Triumphant, Galzomar, or Sikthiss, Maiden of Slaughter on the west side of Auchindoun. Any one of these three mobs can drop the Burning Legion Missive.
Let NPCs Weaken Enemies
When you find one of the three mobs, let the surrounding NPCs engage and weaken it first, then move in to finish the kill at around 50% health for an easier fight.
Kill Only One Demon
Once you kill one of the demons, you are flagged as complete for that spawn — the other two demons that may appear alongside it drop nothing useful, so there is no need to kill them.
Use the Toy
After obtaining the Burning Legion Missive, using it summons a hologram of Archimonde who randomly speaks one of nine different lines, including updated dialogue that was changed to be more lore-accurate, such as references to Kil'jaeden and the orcs not drinking Mannoroth's blood.
