Artist Nils Hamm has illustrated 42 Magic cards, going back to Future Sight. Can you go back to the future set? Never mind. Anyway, he's got five cards in Scars of Mirrodin, and this is one of them.
The first step of the creation of a card's art is its art description. The art description tells the artist what the card will do, what its flavor is, and what the mood of the illustration should be. Here were the instructions Nils was given for Corrupted Harvester:
Color: Black creature
Action: A harvester is a large, horrifying creature that slithers through the Mephidross using its huge scythe to scoop living creatures into the cavity in its back. Show a harvester that has been corrupted by Phyrexia in ways of your choosing -- parts of it have been reduced to bone-and-metal skeleton.
Focus: the harvester
Mood: The terrifying 'custodians' of the Mephidross
The art description also included references to a pair of cards from Mirrodin Block which show what the harvesters looked like before the Phyrexians got to work on them:
Armed with that art description, Nils submitted a sketch to Magic's creative team:
Note that Nils included a couple of tiny humanoids being speared. You know, for scale. Let's see the final version!
And just in time for Halloween, we have the perfect Haunted Hayride!
Here's what the abomination in question looks like in Scars of Mirrodin packs: