Dimir Undead - Kalitas' Zombies (Modern)

So this was intended to be a dinner table special, game night with the friends, just some good fun, turns out, it's pretty good competitively as well.
If looking for a decent modern legal build, the posted deck list should work just fine, plenty of removal/interaction. recurrence of creatures is a moderate theme throughout the deck. Swing and/or block (if able to block) liberally with your smaller, recurring creatures, they'll come back so don't worry about losing them.
Cryptbreaker is a good discard outlet and token generator, also allows you to draw more cards, great toolbox creature.
Heir of Falkenrath is another great discard outlet the gives some decent power in the air, I have however noticed that Wharf Infiltrator does an equally good job, if not better, of cycling through your deck if looking to replace the flyer.
Kalitas is your top-end game-ender. If his lifelink isn't enough to keep up with a super aggressive competitor, then I would suggest bringing in either Vampire Nighthawk and/or Gifted Aetherborn to maintain a healthy life total while building a superior board presence.

if looking for CASUAL play, (not modern legal): Ghoulcaller Gisa is a MUST in this deck! use her ability to sac a token or any other recurring creature, get potentially double zombie tokens. She's GREAT for building a large board threat.

The one Island, while can be frustrating at times, is your insurance against Blood Moon type effects to ensure you can cast your blue cards.