Bogrin Rot Farms
Deep below the streets of Ravnica an intricate and ancient network of sewage tunnels winds its way through the undercity, hoarding the immeasurable waste of the metropolis. This has lead to a vast and complex ecology of molds and fungi, many of which have medicinal, and often illicit, uses. Unfortunately for the guilds of Ravnica the combination of nearly impenetrable darkness, a maddeningly intricate series of tunnels, and predators who have never been seen in the light of day makes harvesting the fungi a nearly impossible task for even the most talented sewer scavenger.
This is where the Bogrin come in. A subspecies of the common goblin, the bogrin have lived below the city as long as anyone cares remember. Blind, navigating only by listening to the echoes of their screeches against tunnel walls, and apparently immune to the rank smell of sewage the bogrin have harvested and grown the fungi as food source for centuries, selling off their surplus to the world above, for the right price.
In the lower quarters of Ravnica you can hear whispered rumors of slime molds the size of men which blindly navigate the deepest reaches of the sewers, grown as beasts of labor by the bogrin, given a spark of intelligence through rituals known only to the eldest and wisest bogrin rot shamans. If you lay your ear close enough to the ground you might just hear legends of gigantic beasts composed entirely of fungal spores devouring anything that delves to deeply into the secrets of the bogrin.