Experiments bring joy, and it is the same for Liches.
Ever since his self-destruction, Amberser had been wanting to rebuild his body.
Even with the help of the Withered Rose, his skeleton wasn't quite the same as before.
But he had been busy all along, and only now did he have some free time to prepare properly.
Ordinary undead creations must use corpses as the foundational material since Dark Magic flows more smoothly through corpses, while the loss of efficiency on other materials is quite substantial, making storage extremely inefficient.
Take the most basic Skeleton as an example. Given the same amount of Dark Magic power, charging a Skeleton for three minutes could keep it clacking about for ten years. But if you switched to a metal body, you'd be looking at charging for two hundred years just for three seconds of movement.
That's why undead can only use magic to enhance their bodies, instead of directly swapping out for steel bones.