What the achievement asks
Open ten Runed Storm Caches around the Siren Isle. Five points, account-wide.
Ten caches, but not ten different ones: the same spot can be looted again once it respawns. That single fact turns this from a hunt into a short circuit.
The caches are invisible until you are almost on top of them
A Runed Storm Cache does not sit in the world like an ordinary chest. It is a faint, translucent shape you will ride straight past.
Two things reveal it. Gusts of wind start streaming off your character when you come near one, and they point the way. Then, when you are almost standing on it, an extra action button called Runecaster's Eye appears. Press it, and the ghostly chest becomes solid and lootable.
That is the whole trick, and it is why players report riding the island for an hour finding nothing. They were passing caches the entire time.
The action button can be pushed off screen if you are already carrying something that uses that slot, such as a crawler mine or a quest weapon. If the wind is pointing at something and you have no button, check that corner of your interface before assuming the cache is not there.
The wind seems to be yours alone. Players have watched someone else trailing gusts and been unable to see the chest it pointed at, so you cannot follow another player to a cache.
Where they are
Fifteen points for ten caches, deliberately. Not all of them are up at once, and the reliable method is to ride the list, take whatever is present, and come round again.
A cache respawns at the same spot after roughly fifteen to twenty minutes. So the fastest route is not to find ten different caches but to loot a handful, keep moving, and come back to the first ones. The cave at 67.30 78.31, the one with the readable tablets, has held several at once.
Bring something that climbs
Two of the usual spots sit on or beside peaks patrolled by Cliffreach Pridetalons and are simply out of reach on foot. An Experimental Go-Pack solves it, and it is worth carrying one before you start rather than discovering the problem halfway round.
Which phase to be in
The caches exist in both the ordinary island and the Seafury Tempest phase, at the same coordinates. Some players report far better luck inside the storm, so if the list above is coming up empty, it is worth switching.
Suzie Boltwrench at 69.13 49.10 moves you into the storm phase.
Where it fits
One of the pieces of You Xal Not Pass, which rewards the Shadow of Doubt mount and is part of Worldsoul-Searching, the War Within meta.
The caches also drop Thunderous Fragments, so this runs alongside the rest of the Siren Isle work: the rares of Clean Up on Isle Siren, the gems of A Choir of Citrines and the weeklies of Siren-ity Now!.