Zombified Spawns are creatures which appear during the fight with Vorkath. During the fight, Vorkath may freeze the player, and summon a Zombified Spawn. The spawn makes its way towards the player, and upon contact, will explode and deal up to 60 damage (30 during Dragon Slayer II), but will unfreeze the player.
Since this spawn deals a lot of damage, it is recommended to cast Crumble Undead on the spawn, as it instantly kills it, unless it ends in a splash. Killing the spawn will also free the player from the bind. Be sure to click elsewhere when you are frozen, as you will continue to attack Vorkath, which can delay your Crumble Undead spell and allow the spawn to damage you. Vorkath is immune to all forms of damage until the player unfreezes.
If Vorkath dies the exact moment that you are frozen, the spawn will not die and continue to follow you. You can however, walk away from it. After a few seconds, it will die without exploding or having to be hit.
Note that if the explosion occurs, it will deal damage based on how much hitpoints the Spawn had left upon explosion; because of this, a risky but alternative method to killing Vorkath is to bring Lunar magic and take advantage of Vengeance, while manually killing the Spawn with either a Blowpipe or Trident, since even if the player fails to kill the spawn, it will usually be damaged enough to make the explosion damage negligible.