What the achievement asks
Find the treasure from 50 Sealed Scroll maps in the post-storm Forbidden Reach. Ten points, account-wide, both factions.
It is the sequel to X Marks the Spot, which asks for ten. Finishing that one and then discovering the meta wants forty more is a rite of passage on this island, and it is the single longest farm in the Dragonflight meta.
How a scroll actually works
This is the part that is not explained anywhere in the game, and it is where most of the confusion comes from.
A Sealed Scroll is an item that drops from things you kill and things you loot on the Forbidden Reach. There are four kinds: Sealed Knowledge Scroll, Sealed Fish Scroll, Sealed Spirit Scroll and Sealed Artifact Scroll. On their own they do nothing.
When you use a scroll from your bags, it marks a spot on your map with an X. You then fly to that spot and interact with what you find there: Mysterious Writings for a knowledge scroll, a Wondrous Fish for a fish scroll, a Token of Blessing for a spirit scroll, a Draconic Artifact for an artifact scroll.
The achievement counts the treasure you retrieve, not the scroll you looted. A scroll sitting unused in your bags is worth nothing, and a scroll you used but never followed up on is worth nothing either.
That distinction catches a lot of people. If your counter looks lower than it should, log through your other characters and check their maps for unclaimed X marks: progress is account-wide, and people routinely find they are a third of the way further along than they thought.
The crystal, and the mistake everyone makes with it
Kraxxus, in the base camp, sells the Azure Scrying Crystal for 2,000 Elemental Overflow. Using one gives you a buff that makes scrolls drop far more often.
The buff is consumed the instant you loot a scroll. Not after a few minutes, not when you leave: on the first scroll.
Read naively, that makes the crystal a one-scroll purchase at 2,000 currency, and that is how most people burn through their Overflow for very little. It is also the whole reason the achievement has a reputation for being miserable.
The trick that turns hours into one hour
There is a way around the buff being consumed, and it changes the achievement completely.
Go to the Froststone Vault, entering at roughly 60.8 37.7 and taking the staircase down. The mobs there respawn almost as fast as you kill them, in a small area, which is exactly what you want.
Use a crystal, then kill for four or five minutes without looting anything at all. Corpses pile up around you, each potentially holding a scroll, and because you have looted nothing your buff is still running. Then loot the whole pile in one go.
You keep the buff through every corpse until the first scroll appears, so a single crystal can yield several scrolls instead of one. People report eight scrolls from one five-minute session, and the full fifty inside an hour with a small group.
Use the group finder. Three players clearing the same hyperspawn is roughly three times the corpses for the same crystal, and groups for this form regularly.
Three practical warnings
Do not farm the War Creche. It is the spot most older guides name and it is considerably slower than the Froststone Vault.
Do not leave the Forbidden Reach with unlooted corpses. They survive a surprisingly long time, half an hour or more, as long as you stay in the zone. Fly away and they are gone.
Beware of phasing. Changing shard mid-farm takes your uncollected corpses with it. If your group phases apart, regroup before you loot.
Paying for the crystals
Each crystal costs 2,000 Elemental Overflow, and you will use several. The good news is that the farm pays for itself: killing the same mobs earns Overflow at roughly the rate you spend it, so people buy four to seven crystals at a time, farm until they run out, and go back for more.
That Overflow also feeds Elemental Overload, which wants 5,000 looted in total and counts everything you have ever earned rather than what you are holding. You will pass that number without noticing.
Two things you will finish on the way
Farming here drops Zskera Vault Keys, which is exactly what Door Buster needs 150 doors' worth of.
If you farm treasures rather than mobs for part of it, each one also counts toward Hoarder of the Forbidden Reach. Treasure farming is slower for scrolls specifically, so it is worth doing only while you still need that achievement too.
Where it fits
One of eleven under You Know How to Reach Me, which grants the title The Forbidden and is a piece of A World Awoken. Of those eleven, this is the one to start first and the one most likely to be the last thing you finish.