The Sorcerer's Apprentice.

There once was a sorcerer’s apprentice, who had studied and toiled for many years in the service of his master, and was nearing the time when he should join the Brotherhood as a full member. So for his final project, his master set him this task: that he should destroy the city of Rome and scatter its inhabitants, by means of vegetation and foliage.
This task sounded quite difficult to the apprentice, and he told his master so; after some thought, the master decided that using the power of a talisman would not be cheating, and he gave the apprentice permission to use one. The master rummaged around in his storage shed for a while, and came up with a piece of deep red crystal about the size of a fist, obviously broken from a larger stone. “This is the last remaining fragment of the ancient Sumerian Amulet of Distress, which was used centuries ago by the magicians of Persia to defeat their enemies. It should be more than adequate to your task.”

The apprentice thanked his master and retired to his rooms to begin preparations for the project. He packed many magical ingredients and sorcerous components: newt’s eyes, dog’s tongues, elf teeth, the hair of homniculi, powdered unicorn horn, dragon scales, everything he thought he might need. He also packed his grimoires and books, his manuals and advisories, instruments and implements, parchment, ink, quills, chalk, candles, vials, a cauldron. He put on a bracelet that would allow him to contact his master telepathically should the need arise. He wrote out his plans in detail, using secret writing and codes to hide the meaning from prying eyes. He loaded it all onto his donkey and began the long journey to Rome.
But he forgot the talisman his master had given him. He forgot the last remaining fragment of the Sumerian Amulet of Distress. Unheeding of the danger, he set up camp near the Seven Hills and prepared his worksite. He started a fire, set up the cauldron and began filling it with ingredients. He chanted and sang. He gestured and danced. He used the entire force of his will and being toward the sole purpose of making the foliage and vegetation grow.

And grow it did. Trees grew taller and bushier, and more menacing. Bushes spread and blocked the roads and highways. Weeds started to invade the aquaducts. Plants everywhere around Rome were growing out of control.
Now, however, the apprentice reached the limit of his power. He had exhausted his own strength and had no more energy to exert. He collapsed in a heap by his campfire. He had overextended himself, used up his own life force in trying to keep the plants growing. He was dying now, and he touched the bracelet, reaching out to his master telepathically.
“Master, what happened? I had everything ready, but the task was still too great. I have failed you and failed myself.”
“My son, are you sure you had everything ready? Everything?”
“Oh no … I forgot it and left it home … the Sumerian Distress Shard!”

“Don’t leaf Rome without it.”