Looking for input; painted myself into a corner in Skull and Shackles

Help! lol

I've made a bit of a big deal about the weird necklace that the Ancient Mariner was wearing, and expanded it into a side-story that one of my players has really taken an interest in. And now I don't know how to finish the side-story!

What I've done is have the necklace be one triangular piece of a six-piece medallion. Throughout the following adventures and travels I've occasionally had them come across another piece. They've learned that the aboleths made the medallion, and now that they have all 6 pieces they have decided not to put them together to see what it does.

What happens is that the pieces are "attracted" to each other, sort of like how the One Ring "knows" it needs to get back to Sauron and somehow it finds its way into hands that can do that. So, the heroes got the medallion from the Mariner, another piece was on the Sahuagin King, etc. The pieces don't detect as magic unless they are brought close to each other. The player who took an interest in this side story is a merfolk.

But what if they do? This isn't something that's part of the module, and I've kicked around a few ideas as to what it's all about but so far I've come up empty. The player wants to find someone to cast legend lore on it to find out what it does but... I DON'T KNOW WHAT IT DOES! lol

Any suggestions?

What levels are the pcs and what book are they currently in? Classes might be helpful.

My first thought was It's a Trap! When it is put together it does something bad. How bad and how quickly is up to you.
- it exudes a slime that turns people into a monster.
- it makes a connection with a "helpful" intelligence. But slowly the answers start corrupting (wait, skull and shackles - so they are likely already evil). The answers are correct, but take the most dangerous form or path.

They're halfway throsugh the Island of Empty Eyes. They've just finished exploring Sumitha and have yet to rebuild the fort there and have the big party.

The merfolk who took an interest in the medallion is a slayer. We also have an aasimar swashbuckler, an undine gunslinger, a human blood rager, and a drow arcanist. (Yeah, my group tends to like the weird races and classes... what can ya do?)

Since the medallion was created by the aboleths, I was thinking about some weird Lovecraftian thing about it. That's why it doesn't detect as magical by traditional means. It's not magic it's just... eldritch.

The ideas I've kicked around is that it gives one control of a shoggoth, hut that's WAAAAY to powerful, even at the levels the characters would be at the end of the module. Even if it invariably corrupts the user, giving this kind of power if far too much.

The other is that it gives you skum to command. Basically, it gives you the Leadership feat, but your cohort and followers can only be skum. The character with the medallion pieces already has the Leadership feat, so I was thinking that if he puts the pieces together, he finds slowly but steadily his followers start to show a strange transformation...

What if it was Aegir's and that's the only way to unseal the Black Tower. :-)

If you're going for Lovecraftian-inspired eldritch relics, the Horror Adventures book has some fun Corruptions to consider. Corruptions are meant to be a GM flavor tool; they're by-and-large negative effects, but come with benefits as well. Maybe adding a corruption onto an effect you previously thought was too good would be a good balance.

I'm actually planning on doing something similar with the deep platinum as you, except in my game, they act like Ioun Stones, and eventually form The Deep Platinum Crown when all brought together. Still don't know what they'll do, though.

Maybe it unlocks a vault that leads to a great Thassilonian treasure: A cannon. Useless for personal combat but it bonds with its owners magically giving them proficiency in it. Perfect for ship combat

Well it could be an ultimate character ending event, the 6 pieces require 6 individuals be in contact when the pieces combine, but on contact, blow apart and are scattered in all directions, miles and miles apart while the 6 unlucky souls are now merged into a new Aboleth, having the base stats of such plus hp/skills of All the now lost victims. In other words, a new villain is born and players now have to roll up new characters, oh, and being an artifact, not even a wish can reverse the event...

Another option is that it could put them on the path into the Ruins of Azlant AP, with their pirates acting as the sponsors whilst the necklace acts as a compass-map to and from that AP's starting point.

If a shoggoth is too powerful, maybe the medallion is a key crafted by a bygone gnomish tinkerer who hailed from Alkenstar, which grants control over his final masterpiece, a cannon golem.