You need to have the centurion quest up to loot the defense map. Unfortunately if you loot the Cache before having the centurion you don't get the map. I have been waiting for 30 minutes now and it hasn't respawned. I'm not sure if they respawn at all.
The defense map quest is shareable, and there have been people nice enough to put up groups under Custom to share it. Keep looking!
IF you're like me and had no idea where to find the cave to turn these rings in, the entrance to find the hand (remember to be in the rift) is at 25.7, 32.5 in the maw (again, in the rift). I wasted a riftkey running around killing things trying to find this entrance .
This needs to be made into an official guide so I can save it on wowhead!
a large or excessive amount of something
"Quartered Ancient Ring is looted from Mawsworn Caches atop the walls surrounding Perdition Hold - These chests can be looted without the Overcharged Centurion, but will not have the ring inside unless you loot them while using the vehicle."can confirm that it drop without using the vehicle and from different caches
In case anyone sees this as of 9.2.5, combine getting Nilganihmaht's Runed Band from Torglluun and then finishing the final stage quest, Gotta Hand It To Ya. It makes it much easier due to only needing to use one Repaired Riftkey. The last quest, Gotta Hand It To Ya after obtaining the 5 rings, requires you to go into another rift. The Repaired Riftkey were kind of annoying to farm, only getting one while farming rares.You can use a Cypher of Relocation to get back from the maw quicker after entering a rift within Koria. Alternatively if you have Shadowlands engineering, Wormhole Generator: Shadowlands and select the Maw, it takes you right next to the rare in the rift Torglluun for the Nilganihmaht's Runed Band. Final quest, Gotta Hand It To Ya is right next to him as well.