The witch slaves are dead trolls, reanimated, and taken control of.
These trolls guard the witches until either the troll or witch have fallen. They aren't intelligent enough to use weapons, but instead rely on their brute force and heightened senses (this happens during the reanimation process) to take down their foes.
Trolls come in all shapes and sizes. Their greatest passive weapon is also their greatest weakness. By having heightened feelings, the trolls are less resistive to pain. With heightened sight a simple flare spell would work, and their heightened hearing would make loud noises a plus for us.
The Witch slave was first discovered by a magician working as a spy against a witch for King Artorius (King Arthur). He played the role of a necromancer at her disposal, and he saw how she raised these trolls from the peak of mountains worldwide. The most preferable would be the Rocky Mountains and the Himalayas.
Soon after the magician wrote the note to the King stating what he saw, a spy against the King for the said witch led to the magicians' death and all of the dark witches in that hemisphere to attack and kill the King. Just as he was about to die, he threw the sword of Excalibur into an ocean and the item was never found again.