What the achievement asks
Kill the last two bosses of Ny'alotha, the Waking City: Carapace of N'Zoth and N'Zoth the Corruptor. Ten points, account-wide, both factions.
They are the eleventh and twelfth encounters, gated behind the whole raid. This is the end of Battle for Azeroth.
The legendary cloak, and why the queue can vanish
This is the trap that catches people coming back for the meta, and it is not documented anywhere in game.
If Ashjra'kamas, the legendary cloak from patch 8.3, is not in your bags, the Raid Finder wing for The Waking Dream stops being offered. The option simply is not there when you talk to the queue NPC, with no message explaining why.
Plenty of players put the cloak in the bank once it stopped being useful, then came back years later and found the wing missing. Taking it back out of the bank restores the option immediately.
Check your bags before you queue. If The Waking Dream is not on the list, this is almost certainly why.
How the raid divides up
- Vision of Destiny: Wrathion, Maut, The Prophet Skitra
- Halls of Devotion: Dark Inquisitor Xanesh, The Hivemind, Vexiona, Ra-den the Despoiled
- Gift of Flesh: Shad'har the Insatiable, Drest'agath, Il'gynoth
- The Waking Dream: Carapace of N'Zoth, N'Zoth the Corruptor
The Waking Dream itself can be earned on any difficulty, Raid Finder included. The parent achievement Ny'alotha, the Waking City is what the meta counts, and it asks for Normal difficulty or higher.