The Undeads were a blasphemy against the goddesses. Even Ganondorf didn't know the origin of the monsters, the one cavity in his knowledge, and it was this mystery that so fasinated him. He had captured one himself, a pale and leathery thing that moaned and screamed in its steel cell, and experimented on it endlessly. Its weakness was light, whether it come from the sun or fire, but nothing destroyed the atrocity. From light it shriveled and collapsed in a helpless mound, but come the nightfall it unfolded back to its original form. A weapon could smash its bones, but within hours the broken limbs slid back into place and the howling started again.
With the corpses of the Hylian guard, Ganondorf began his first tentative attempt at creation. The naked bodies were hung on meat hooks and drained of whatever blood hadn't escaped at their deaths. Next the organs were removed and destroyed, with the exception of two. Ganondorf had discovered small, shivelled lumps of flesh within the chest of his captured Undead and had guessed that they were perversions of the human heart and lungs. But how to imbue the thing with movement? Here was the paradox that impeded Ganondorf's progress. The Undeads were certainly not alive by any standards and yet they had conquered over death and remained functional. They were no simple spirits, ghosts were the residue of souls that had been unable or unwilling to move into the realm of death, but rather husks of humans that had been able to grasp onto some aspect of humanity and thus continue to exist despite the laws of Life. Had he not see them with his own eyes, Ganondorf would have dismissed the Undeads as inconcivable.
The mutilated cadavers filled the castle with the lugubrious stench of death - the horses shrieked and rolled their eyes until their ropes broke and they escaped and his Gerudos mumbled prayers and carried sacred candles when they walked their rounds - but Ganondorf could not stop his desecrations. The accursed book by the mad king Addorin had taken days to open and when Ganondorf had finally broken the spells around it and flipped through its yellowed pages, the imprisoned Undead chuckled