Comprehensive List of confirmed Sources of Writhing Essences:
(they only drop after you've accepted the quest in Dalaran)
- Weekly Mythic+ Cache (20)
- Emissary Caches (3-5?)
- Paragon Reputation Chest (A chance for 3-5)
- Tomb of Sargeras Raid Bosses once per Week not on LFR (varies)
Confirmed Sources that do not drop Writhing Essences:
- Mythic+ regular Rewards
- Legionfall Chests
- World Quests
- LFR
- Normal / Heroic / Mythic 5 Man Dungeons
- Normal Arena / BG Rewards
Feel free to contribute if you know something.