Giantborn fungus

Giantborn fungi grow exclusively on giant corpses. They display a pale blue aura and are widely known as the easiest source of magic that doesn't rely on Giant bones. Despite its name, this plant is not a fungus but a type of vine, whose flowers look strangely like parasitic mushrooms, hence the initial misclassification.

Ethereal plant

The vines are connected directly to the bones, without ever touching the ground. Yet, they have no visible root and instead look like they just wrap around the skeleton, growing without any means to acquire nutrients. In fact, their roots are not in the physical world, but wherever the lingering magic in the giant bones resides. They feed on the supernatural energy tunnelled by the giants, millenniums after their death, and use it to grow to unimaginable size.

While it is unsure if the resources of the corpses are limited or not, the plant remains a simple and efficient way to harvest and bottle magic, far easier and more sustainable than sampling giant bones. Should they be removed from their source of alimentation and have their roots cut off, they will wither and die in a few days, making the harvest a delicate and time-limited task.

Gardeners

At least one Gardener is assigned to each giant skeleton over the world. Their task is to routinely harvest the vines to renew the world's magic stocks, and to weed them out if they cover the corpses too much. The plant completely enveloped the bones for millenniums without causing any issue, but now that they are a pillar of the modern world, nobody wants to risk them damaging the only source of magic. The pool contained in the bones may also be unfathomably large rather than infinite, but there is no way to know for sure.

Of giants and magic

Giants were a race of individuals as tall as a hill or a mountain, which roamed the world millenniums ago. Nowadays, their skeletons litter the world, the titanic swords they wielded by their side. It is said that they disappeared in a self-destructive war against outer gods, pushing the invaders out in their last breath. They also possessed the power to bend the reality to their whim through a phenomenon known as magic, which requires a poorly understood energy that seeps only through their bones.