There are mainly two sources.
The first is natural evolution, like Skeletons, Zombies, Ghosts, Shadows, and Specters.
No human intervention is needed. In some unique natural environments, such as areas rich in Negative Energy or regions filled with resentful death, they may arise.
This type of Undead often follows its original biological characteristics.
Human corpses can only transform into Regular Skeleton Soldiers or Zombies and will not become Giants.
The Skeleton Soldiers transformed from Trolls are also large-bodied.
Even if human intervention occurs, it only increases the transformation success rate, and cannot easily change the form.
The second type is artificially created, like Death Knights, Witches, Death Stalkers, Corpse Hounds, including Reaper Demon Lords, which belong to this category.
They cannot be born naturally; they must go through complex spellcasting ceremonies, requiring a suitable transformation subject to summon them, both aspects are essential.
