Demon Ivan has his eyes set on human Alfred and he usually gets what he wants. Plot's up to you as are any other characters or pairings is fine but a heavy dose of Russia/America is required.

Bonus: Arthur is the one who first summons Ivan (accidentally or on purpose is your choice)
Bonus 2: Ivan's full demon design is based on something like this
http://24.media.tumblr.com/daaf3a25d043cfe11f7deb7d6ea4bbd4/tumblr_ms7idxzWcr1rc5rrso1_500.jpg