These magic items figure heavily in Arc H - The Lost Soul of Curse of Strahd: Reloaded's second act. When I was doing the research into hag's heartstones, I didn't know that there was a real thing as a witch stone, also known as a hag stone or an adder stone, that figured prominently in the folklore British Isles. Typically found by water, they're pebbles or rocks with holes eroded through them, often turned into talismans to ward off bad luck. Going down the rabbit hole further, I discovered prophecy stones, which are arguably even cooler and more fractal-looking. I split the difference between the two, and came up with the heartstone for the Bonegrinder coven.
The bottled soul leech was inspired by the creatures from James Cameron's film The Abyss - something primordial and slightly embryonic.