Long story short, I was reading over sliver lore on the wiki and made a connection that I thought I'd share.

As per the wiki, the planar origin of slivers as a species was an unnamed plane where an unknown sentient species built their empires by using slivers as beasts of burden. This ended when the Sliver GraveMother evolved to conquer death and the result was a cataclysmic night where sivers overran the population and killed anything that wasnt a sliver on the whole plane.

Long after this, Volrath would visit the plane and take a liking to the species. He'd capture the queen and some of the babies and bring them back to Rath for him to experiment and keep under the Stonghold. The Weatherlight stuff happens, the Rathy overlay happens, and all the slivers end up in the volcano in Urborg which kills the queen and the rest die during invasion.

They're later revived without the queen by the Riptide Project in a bigger blunder than Jurassic Park where they immediately escape the labs and consume everything on the island. By the time of Time Spiral they're an unbridled disaster for everything as without the queen to lead them they are just feral beasts driven to eat and reproduce. This is where Hivestone see print.

"Creatures you control are Slivers in addition to their other creature types."

*"Rath's evincars used hivestones to control the slivers, but the overlay put their power into unwitting hands. Now the stones make not masters, but slaves." *

This is an amazing call back to the lore, and generally all the slivers with flavor text in this set are loaded with details.

Here's where things get interesting. During the events of the Weatherlight, Karn manages to get the Legacy artifacts by talking to the Sliver Queen and comparing his relationship to the artifacts to her relationship with her offspring. This implies that, on a very accessible level, the Sliver Queen is fully sapient and capable of reason, emotional intelligence, and empathy. Something that texts from after her death show is almost diametrically opposed to slivers without her.

Part 1, I believe that is what the Sliver Queen brings to the hive. A sapient, fully in control, intelligent and emotionally receptive mind. The single guiding consciousness of the hive. With her death, the slivers are nothing more than instinct driven animals. In the same way that each sliver grows wings or claws when a sliver of that type comes near, the presence of the Queen suplicates the personality if the hive mind across the population.

The flavor texts state that the Evincars used the stones to control the Hive. How better than tapping into it as a fellow sapient being and either talking with it or maybe demonstrating it psychically. What's the difference between when the Evincars of Rath used the Hivestone and now? It states right in the flavor text that the Hivestone was put into unwitting hands during the Overlay, the exact situation that killed the queen.

I propose that the Hivestones function as a direct psychic link to the hive mind, and the Evincars used it to communicate with the conscious mind provided by the queen. With her death, the Hivestone still connects to the Hive mind but now it's an unending echo chamber of base instinct and animalistic needs. Where before you could either communicate or maybe dominate a single mind or control, now you're engaging the entire swarm on a psychic level that they are naturally part of. It turns everything into slivers and enslaves their minds because anyone that uses it will get over run mentally in the same way they operate physically.

Lastly, I think the Hivestones are actually older than the stronghold nest. Volrath visited the home plane of the slivers, he likely saw the ruins of the civilization that they destroyed. What if the sliver stones are how they managed to domesticate them? The bit and bridal of a culture that tamed them. He could have used them so locate and transport the slivers back to Rath without a fight as opposed to somehow overpowering a sliver hive in combat. Now, eons after they were made and centuries after they were discovered, they no longer function as a tool for communication because the Hive has outgrown the power of those that could use them.