“Our babies are born at an incredibly early stage—but instead of moving to a pouch to develop, they're implanted into a hive, where they can finish developing and grow to a proper size.”
Troll Hives look loosely like a giant gray beehive, and are filled with green jelly that has soft, white, glowing, tiny bubbles. This solution is kept at 150 degrees, and reminds Sophie a bit of Aloe Vera gel. They are used in Flashback to save Luna and Wynn's lives. Each compartment in the hive has a control panel with many buttons which can be used to close the compartment and fill it with the jelly, as revealed by Fitz and Biana after they lock Alvar in one.
Several thousand years before the books take place, the troll hives were raided by ogres. The trolls managed to fight them off and maintain control of their lands, but they lost an entire generation in the process.
To prevent this tragedy from repeating itself, Luzia Vacker helped them to hide their hives with illusions at Everglen near the lake. When more space was needed, they moved the hives to Dawnheath. A troll hive was discovered in Flashback to still be on the property of Everglen, covered by Luzia's illusions. The trolls from the hive destroyed much of the property. Later in Flashback, it's revealed that Luzia had another hive hidden near the overdrive where Luzia and Empress Pernille were secretly trying to design trolls that were both smart and strong as newborns.