Scavenger of the Forbidden Reach

For this achievement you may open 20 of any of the treasures needed for Treasures of the Forbidden Reach except for the Forbidden Hoard, which there is a separate achievement for. You can find the treasures all over the Reach. I found the War Creche to be a good spot for them as there are 4-5 close Obsidian Coffer spawns, but the area is also packed with hard hitting Elite mobs so this may be a struggle for some classes.

Once finishing this, you'll need an additional 80 for Hoarder of the Forbidden Reach if you're going for the zone wide achievement.

Edit from live: The [item=204558] purchased from [npc=200566] will mark these treasures on your minimap for 1 hour, making them significantly easier to find.

I purchased and used the Reliquary of Perception and my average was about 3 caches per hour. This is going to be a long haul.

If you are doing You Know How to Reach Me for A World Awoken, here's some things I noticed while doing it:

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Treasure achievements: Treasures of the Forbidden Reach + Scavenger of the Forbidden Reach + Hoarder of the Forbidden Reach

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• [object=386174]
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• [object=386165]
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• [object=386179]
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• [object=385520]
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• [object=386168]
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• [object=386167]
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• [object=386208]
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• [object=386212]
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• [object=386213]
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• [object=386214]
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• [object=386172]
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• [object=384763]
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Scroll Achievements: X Marks the Spot + Scroll Hunter

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• [item=202669]
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• [item=202667]
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• [item=202670]
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• [item=202668]
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Note: You only get credit towards the scroll achievements after you have used the scroll and picked up the "special treasure".

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First of all: I recommend combining doing the treasures and scrolls at the same time!
I noticed I was getting more scrolls from treasures than killing things with [item=204560] buff.

Second of all: "Aim for the last achievement".
This is especially for the treasure achievements. Basically don't start looking specific ones for Treasures of the Forbidden Reach and instead just start looking for any treasures for Hoarder of the Forbidden Reach.
There's a high chance you might just find all you need for the first achievement by just going around and picking any treasure. Only start looking for the specific ones for the first one after you're done with the bigger achievement, if somehow you're still missing some!

TLDR: First aim to complete Scavenger of the Forbidden Reach, then Hoarder of the Forbidden Reach, and only after that one start looking for the missing ones for Treasures of the Forbidden Reach.

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BUFFS

• Doing Into the Storm should give you plenty of [currency=2118] (these are warbound/accountwide, so make sure to transfer all from your alts too!), which you can use to buy [item=204558] from [npc=200566] to help you find treasures, and [item=204560] from [npc=200563] to help you find scrolls.

/way #2151 34.2 59.8 Cataloger Daela (Reliquary Scroll of Perception)
/way #2151 34.0 59.8 Kraxxus (Azure Scrying Crystal)

• Don't buy too many of [item=204558] as the buff lasts 1 hour. I'd recommend only buying 2, so basically use one and have one as a backup in your bag, and only buy more if you need more.
  This buff will make the treasures appear on your minimap, and make sure to stay bit closer to the ground while flying around, because if you are too high, your buff might not detect the treasures.

• While you do need 50 Scrolls in total to finish both Scroll Achievements, don't buy too many of [item=204560]. Just start with 10 or less, you can always go get more. Note the scroll buff is consumed when you find one, so you need to use another to have the buff again!

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Additional note:
Be sure to open your whole big map as well to check occasionally, if any [object=384763]s for Forbidden Spoils achievement are around, and if Froststone Vault Primal Storm is active for Under the Weather achievement.

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Good luck! Be sure to take breaks in between too!