Zombies are undead creatures that lurk The Crypt. They are very slow, but poisonous, making them trick enemies when fighting in groups. The most defining characteristic about them is their inability to be killed by normal means, because they can get back on their feet after being knocked out. The only way to truly defeat them is by impaling their chest with wooden stakes, which can be found scattered trough the crypt, or bought with Maria.
Zombies can also be summoned through the Raise Dead spell, both by players and liches, so their numbers are, theoretically, infinite. What awakened them from the afterlife is not truly know, though it possibly has been some relation with the Cult of Akbaa.