This achievement is for collecting all 5 Tomes of Resurrection.
Tomes of Resurrection are the most recent and final set of tomes you will have to collect. Tomes of Resurrection are found within Ashen Chests that require Ashen Keys to open. There are 3 main way of obtaining Ashen Chests, one of which guarantees one of the Tomes of Resurrection.
You can get chests from Skeleton Fleets or as a random drop from Meg or the Kraken and keys from fishing, but I am concentrating on the ones you can farm with the smallest amount of waiting time between islands/events.
Guaranteed Tome of Resurrection... MaybeAs of this edit I have not been able to get a skeleton fort at Molten Sands since the March 2020 update to see if you're guaranteed a Tome or Resurrection upon completion. Also, who wants to do that fort? No one, that's who Rare. If you see Molten Sands is the active fort it is worth a try. If anyone who reads this happens to complete a Molten Sands fort, please leave a comment as to whether or not you received a Tome of Resurrection from the Ashen Chest found inside the vault/loot room upon completion post March 2020 update.
The following two ways are RNG based so good luck.
Easy & FastIf you have doubloons and/or gold to spare you can go to Larinna outside any of the taverns in the world and purchase an Ashen Chest Stash and Ashen Key Stash voyages. These will give you a X marks the spot map for the outpost you're currently located. Dig them up and role the RNG dice.
Check Larinna's prices for yourself on the Stash voyages as Rare has been changing the price between updates as of this solution edit.
Hard & Time ConsumingIf you want/need to save your doubloons and/or gold then sail around the world looking for skeleton captains. These will be either Ashen Guardians or Ashen Key Masters.
Guardians drop a riddle order that you will have to complete to obtain the Ashen Chest while the Key Master drop an X marks order for the spot for the Ashen Key. The orders drop next to the Villainous Skull from the skeleton if you are having trouble locating it. Typically the orders are for the island you are located on.
Time Saving Tips
1. Ashen Guardians hold swords while Ashen Key Masters hold flintlock pistols. If you need keys because you have lots of chests (or vice versa) then this will let you utilize your spyglass to save you time from having to anchor or jumping off the boat to determine which Ashen captain you're looking at.
2. Ashen Guardians and Key Masters spawn in predetermined locations on every island. The complete list of these spawn locations for each island is a long list and this text heavy solution is enough of a read as it is, so I will give one as an example.
Cannon cove has a beach on the east side. The Ashen captain will spawn on this beach towards the south stretch of the beach.
2.5 This is something I noticed while doing the Thieves Haven Athena Run voyage and sitting on Thieves Haven while my crew sailed to the riddle islands for 7 hours one day. The longer you remain on an island the more locations on an island Ashen and regular captains will spawn. The spawns are always close or on a beach.
3. Rare keeps changing things in the game (mostly for optimization and visual continuity between the Xbox One consoles and PC). Because of this they ashen captains have had several visual changes since this solutions original March 2020 posting. Originally ashen captains looked like any other emergent captain only with a red glow. Now they look more... asheny... than normal captains. Hopefully this is their final iteration.
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Thanks to thatNoseyParker and ImmenseCookie for informing me of changes so I could update the solution.