Protecting Mermaid Pearls

Question

Since the dawn of time, mermaids have been able to make pearls through magic. (Mermen create seashells, and these are what the mermaids wear.) These pearls form in the center of their seashell top, right over their heart, over and over throughout their lifetime, and hold a bit of the mermaid's essence (they literally put their heart and soul, part of themselves, into their pearls. Don't worry though, what they lose they eventually regain since the heart and soul are immortal.)

Since each pearl contains a piece of their self, mermaids give them to their loved ones as a sign of their bond; for spouses, this strengthens their marital bond, while for their children, it brings out their potential and allows them to feel their mother's love for them within their own hearts. This is even more touching when one considers that a pearl gives one power over its creator; if you have a mermaid's pearl, you have power over them.

However, now that Atlantis has removed its Barrier of Secrecy there's a problem: everyone wants their pearls. Human men seem to sense the significance of the pearls and desire one (or more, there are hoarders) for their own, and if that wasn't bad enough, they have an uncanny knack for finding them. Additionally, witches have realized that they can claim a mermaid's pearl, adding her magic to their own and getting a new slave as well. They (and other mages as well) also want to claim the pearls as ingredients for their potions or spells, for twisted experiments, and who knows what else. Sea lamia (yes, there are sea lamia, just as there are sea snakes) desire the pearls for use in subjugating the mermaids, which they are incredibly jealous of (humans like mermaids better, so they feel wronged and want revenge). Even crazy dragons want these pearls! (see ("https://worldbuilding.stackexchange.com/questions/193057/how-dragons-can-hoard-people") if you're curious as to why)

So now the question is: How to defend the mermaid's pearls?

Consider:

1. While the deep-sea location of Atlantis is a significant barrier, there are spells that negate even that advantage. (Magical submarines are tricky to make, but they are a thing.)
2. A simple "force field" can be broken or bypassed, so that's not a solution. Something more restrictive, like the Age Line from Harry Potter and the Goblet of Fire would be preferred.
3. Since no defense is invincible, and no defenders can repel would-be pearl thieves forever, a secretive location is best.
4. Secrecy, as mentioned above, are your friend, as are complexity and redundancy. You're designing a magical security system here, and stakes are high if it fails.
5. While magic exists, powerful magic is rare and powerful enchantments are even rarer. Setting up something like the Ark of the Covenant, that'll disintegrate anyone who's dumb enough to open it up, is not possible. Neither is impenetrable defenses or an impossible maze. Everything magical has to have weaknesses; the more "unfailable" you make it, the less likely it'll work. Simple but effective is the key here.

Answer 1

Security through pearls

I'm assuming mermaids can make multiple pearls.

Although the pearls seem to be a weak point, the pearls can also strengthen them. First, set up barriers that can only be accessed by pearl holders. Second, have teams of reconnaissance scout for submarines. Submarines have many weak points, and either physically or magically tampering with it can be incredibly deadly to the occupants. They are limited in their movement and require a lot of spells to survive different pressures, refresh air, get food and in the end manipulate things outside the sub to get the pearls or possibly take down a barrier or something.

These two can already foil most of the attempts. No security is perfect, but that is never a reason not to have it. Locks and barriers will reduce the amount of people who are able to get to the valuables, while in most cases also increasing the time. Even if people can circumvent the security, the perceived trouble can already deter people as they might think it's not worth it.

The requirement for pearls to pass the barriers is important. Not only because it'll require something specific to merfolk, but also because it'll strengthen the people. If these pearls are the only thing that can help you go about, the merfolk are forced into tight nit communities that will look after each other. They will have strength in bond and numbers. You can potentially increase security with two factor authentication, requiring the merman magic in some way to pass the barriers, forcing each team to have at least one female and one male. This will further increase the numbers and strengthen the bonds, as you're again dependent on each other. No easy sneaking about and accidentally getting captured of youngsters for example. Although they might have more difficulty getting back in the safe zone and more might be captured because they lost one of them in one way or another, the general population is more safe. Checkpoints where people inside can open and close the barrier for outsiders can help reduce this problem. This can also mean that for smaller sorties you can leave pearls at home.

As the soul and the pearl are linked, you might even be able to detect a stolen pearl due to distress of the creator, allowing the barrier to attack or ignore people trying to pass it, even if they can find a merman as well to aid them. All this so basically the only way is to break the security another way is more plausible, allowing only the most knowledgeable and powerful to get past these defences.

Answer 2

This isn’t as difficult as you’re making it out to be.

You don’t need an impenetrable defense, a reasonably good one will do.

The ocean is vast, and if magical submarines are anything like ours, they can only search so much territory before surfacing, and the pressure proves problematic. In our world, submarines are relatively common, yet we know less about the bottom of the ocean than we do about the surface of the moon.

In this world, if submarines are rare, you can basically ignore the prospect of any surface dweller finding a pearl, so long as the hiding spot is well camouflaged in deep water.

This is less the case if mages can somehow sense the presence of pearls, but if this is the case, there probably exists a way to mask that scent, so to speak.

That just leaves other aquatic races, like the lamia, to deal with. Once again, the odds are on your side. Unless there is some unblockable way to sense the presence of pearls, an enterprising sea lamia has no clue where to start looking, and the ocean is far too vast to scour.

Aside from dumb luck, which can probably be prevented via very simple illusion spells, or even just a safe purchased from the surface dwellers, the only way a lamia could realistically find a pearl is by following a mermaid and watching her hide it.

To deal with this, the mermaids could take some tips from the crows: craft fake pearls, and hide those. Once the lamia is distracted, swim off and hide the real one.

Another note: the force fields that you said weren’t an option because they could theoretically be bypassed? Walls can be bypassed, too, so why bother to have those? Locks can be broken, so why use them? The average lamia probably isn’t going to carry a full set of force-field breaking spells on them at all times, and if a lamia does have such things on their person, the odds of them stumbling on a pearl are slim.

You could set up a force-field and a ward, such that the mermaid instantly knows if her pearl’s hiding spot has been discovered. That way, she can re-hide the pearl before the lamia comes back with a force-field breaking team.

Also, periodically making and hiding fake pearls would further discourage search parties, due to the prospect of wasting a ton of time and energy breaking a force-field only to find it was all for nothing.