You will make creatures from myths and legends. Mtg uses a lot of mythical creatures like minotaurs, harpies, dragons, etc.
which are mostly from Greek or European myth. If you want to use those that's fine but I encourage you to use creatures not already used by mtg, like from South America or Ancient Egypt. It would be even cooler if it was from North American myth. These will be more likely to win. If you need inspiration, I have plenty of ideas. It might be hard to find art for some of these, but I believe you can even if you have to search through the deepest, darkest recesses of the internet. The card's abilities should somewhat match the myth.
Contest will end when I feel like ending it.