Artist Nils Hamm has illustrated 28 Magic cards, going back to Future Sight and Tenth Edition.
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 given to Nils for Wretched Banquet:
Color: Black spell
Action: This spell represents the thirst of the wretched vampires of Grixis. There aren’t many living creatures on Grixis, so vampires there have had to feed on other undead creatures, making them bony, wasted husks. Show one of these wretched, starving vampires sinking its fangs into the neck of a limp, fleshy zombie. Running out from the wound, instead of blood, we see rivulets of blackish-greenish ichor.
Focus: The bite is the most important part. Feel free to "zoom in" on or otherwise highlight that action.
Mood: We all do what we have to, to eat.
Here's the initial sketch that Nils submitted:
Wretched Banquet initial sketch by Nils Hamm
Okay, that's a little more sketch-y than what we usually show, but it looks kind of neat. Let's skip ahead to a more completed version.
Wretched Banquet second sketch by Nils Hamm
This version clearly shows a ravening vampire, but the creative team wanted to make a few changes to the victim. It's more interesting if the vampire isn't feeding on a helpless pale waif, which vampires have been doing constantly since Bram Stoker. In fact, if the victim is a rotting zombie, that will show how hungry the vampires of Grixis must be.
Wretched Banquet final art by Nils Hamm
Here's the card in its final state, as you'll see in booster packs of Conflux.