Unholy Grotto

{T}: Add {C}.
{B}, {T}: Put target Zombie card from your graveyard on top of your library.

Zombie Master

Other Zombie creatures have swampwalk. (They can't be blocked as long as defending player controls a Swamp.)
Other Zombies have "{B}: Regenerate this permanent."

Death Baron

Skeletons you control and other Zombies you control get +1/+1 and have deathtouch. (Any amount of damage they deal to a creature is enough to destroy it.)

Grimgrin, Corpse-Born

Grimgrin, Corpse-Born enters tapped and doesn't untap during your untap step.
Sacrifice another creature: Untap Grimgrin and put a +1/+1 counter on it.
Whenever Grimgrin attacks, destroy target creature defending player controls, then put a +1/+1 counter on Grimgrin.

Noxious Ghoul

Whenever Noxious Ghoul or another Zombie enters, all non-Zombie creatures get -1/-1 until end of turn.