Mindleech Mass, a servant of the blue-black House Dimir, roams the undercity, literally feeding on intelligence of what goes on in the surface world. In game terms, that means it lets you pilfer your opponent's hand and commandeer a creature or other spell that you find there for your own purposes.
In flavor terms, it's literally a creature made of mind-sucking leeches. Behold:
Mindleech Mass art by Kev Walker
See him reaching up to the tunnel ceiling to drain some intelligence from the surface-dwellers above? See how he's one big, animated glob of slimy leeches? Ughhh.
Now you know why most Ravnicans stick to well-lighted places and try to forget about the dark goings-on below them. You can find more art of the undercity in the art of cards like Golgari Rot Farm, Duskmantle, House of Shadow, Dimir Aqueduct, and several of the Ravnica Swamps.