To start with, a Walking Ballista on the battlefield can stop the Saheeli Rai / Felidar Guardian combo, which will make it valuable to many decks to start the format
I learned to prioritise removal, card draw, and a clear win condition
I'll let you know how each deck works, then I'll give you some options to upgrade it once you've built up your collection more
3) Season of the Witch from The Dark Artist Jesper Myrfors What would Halloween be without witches
Commander (formerly EDH) is a Magic: The Gathering game type where each player plays with a 99- cards deck, led by a Legendary Creature: the Commander
Most importantly, giving your demon deck that would otherwise just be heavy on black access to blue is bound to be useful, giving it access to stronger draw spells and counterspells