Once per turn: You can send 1 monster from your hand to the Graveyard, then target 1 Level 4 or lower Zombie-Type monster in either player's Graveyard; Special Summon that target. This card must be face-up on the field to activate and to resolve this effect.
Can steal monsters from your opponent’s graveyard.
Discarding a monster can be costly.
Can only revive low-leveled monsters.
You can revive the monster you used as cost. But you must have at least one applicable target in either graveyard to activate the effect and therefore the discard cost.
You can revive another Zombie Master. The newly revived Zombie Master can then revive another monster. Combined with the tip above you can discard a Zombie Master from your hand and then immediately revive it.