Flying, trample
At the beginning of your upkeep, sacrifice a non-Zombie creature, then create a 2/2 black Zombie creature token.
If Archdemon of Unx is the only non-Zombie creature you control when the triggered ability resolves, you’ll have to sacrifice it. You’ll still get a Zombie token.
If you somehow control no non-Zombie creatures when the triggered ability resolves, you won’t have to sacrifice anything. You’ll still get a Zombie token.