The Heartstone of Thorns

Long ago, Veridora was a city known for its arcane prowess and pursuit of forbidden knowledge. Its inhabitants delved deep into ancient rituals and experimented with powerful magics. Seeking to gain control over the natural world, a group of ambitious sorcerers performed a forbidden ritual within the sacred depths of the temple.
In a chamber beneath the temple, dedicated the worship of nature, the sorcerers poured their magical essence, into a large rock, as descriped in an ancient book. After multiple hours of immense concentration the group of sorcerers were exhausted, but to their delight the once dull stone, was now beaming with a red glow. To attune it with nature they placed it in a cradle of vines.
Unbeknownst to them, their reckless actions had unintended consequences. The ritual tapped into an ancient source of primal energy, a raw essence of nature itself. The sorcerers, blinded by their thirst for power, sought to harness this energy for their own purposes. They believed they could manipulate and control it to reshape the world according to their desires. It was this desire that caused the magic within the stone to become corrupted and evil.
While the sorcerers where away, resting after their ritual, the vines wrapped around the stone. The magic of the stone corrupting the vines running through the city. These corrupted vines spread to various animals in and around the city, and in the middle of that very night, the city was attacked by a horde of mutated vine-like creatures.
The sorcerers recognizing the source of this magic as their own, hurried to the chamber to destroy the stone, but only ended up destroying the chamber around it through their fight with the vine monster the stone now had become.
Only few inhabitants of Veridora escaped and in a desperate attempt to contain the evil within they sealed the entrance of the city using ancient magic.