My world is a medieval age with magic setting, with a fantasy style map (assume flat earth. Not entirely true but accurate enough). The world is several times bigger than Earth.
An Empire which controlled quite a lot of this world some 20,000 years ago had a huge, undefeatable army which it supported by growing crops on a massive scale using magic and feeding them. However, a magically engineered plague wiped out most of the plants and made it impossible to grow anything for ten to fifteen years. As a result, 80-90% of the population starved and died.
When the seeds which had been stored years ago were finally magically treated to be immune some ten years later (this magic hadn't been developed until then), this period, called the Great Starvation, finally ended.
How would economy and culture have been affected during and after the great starvation?
(The population is around 70 Billion, with around 28 Billion out of them slaves and 42 Billion 'Citizens')