The Emerald Nightmare: Il'gynoth

Be aware that you can get the mission Facing Our Fears if you already have accepted but not completed the quest Into the Nightmare: Il'gynoth. When your followers complete the mission you will receive a message that you already have this quest and you will receive a second Corrupted Egg Fragment in your inventory. After killing Il'gynoth and turning in the quest, both Fragments will be removed but you will only receive one Cache of Nightmarish Treasures.

tl;dr If you have Into the Nightmare: Il'gynoth in your quest log don't complete Facing Our Fears or you will lose a 2nd Cache of Nightmarish Treasures

Confirmed: I received this quest in my class hall. Killed the boss in LFR mode, and got it completed. You can kill Il'gynoth in any difficulty to get the chest, that will be scaled to that difficulty. Mine was an LFR chest.

Received this quest from my mission board. After accepting the quest, queued for LFR Darkbough RAID. Although I had completed the raid earlier in the week, was still able to queue and enter LFR to kill 2nd boss to complete the quest. No need to finish the RAID if you already finished because you get NO LOOT if already completed. Very quick, very easy.