What the achievement asks
Open ten Mawsworn Caches during the Necrolord assault on Perdition Hold. Five points.
No quest required, which makes this one of the three dependable pieces of On the Offensive. If the Necrolord assault is live, you can do this.
Where the caches are
Ten will take more than one day
There are not ten caches on the ground at once. What saves you is that they reset with the daily quests, including within the same assault, so a Necrolord assault that runs three and a half days gives you three passes at them.
Some caches also refuse to give credit, apparently at random. That has been reported since 2021 and again in 2024, so budget for it rather than assuming you miscounted.
Getting up to the awkward ones
Several caches sit on top of the walls and towers of Perdition Hold, and this is where people stall.
The grapple points need the Animated Levitating Chain from Ve'nari, 1050 Stygia. It is also one of the thirteen upgrades that count toward 'Ghast Five, so it is not a purchase you waste.
The Overcharged Centurion reaches everything, including the caches nothing else can. Mikanikos builds it for you after the quest "Putting A Plan Together", which is itself a follow-up to "Waiting in the Wings" and therefore subject to the same random quest pool as everything else in these assaults.
Without either, people combine Borr-Geth's Fiery Brimstone with a Goblin Glider Kit to reach the towers. It has to be the glider kit specifically: slow fall effects and other parachutes have been tested and do not carry you far enough.
Opening a cache while inside the Centurion has been reported to sometimes give no credit. If a cache does not tick, step out and try the next one rather than repeating.