I have a setting where the cycle of life is well known and proven. A creature must be born, grow old, and die. After that, its soul goes to the Otherworld, and will born again.
The world is built upon dualism: there's a god of light and a god of darkness; mother and father; birth and death; etc. (19 gods, with one almighty force included). Their existence is also well known and proven, but their assumed power varies culture by culture.
Now I have a lot of story ideas involving undead creatures, but I can't explain how, and what types of creatures could exist. It is certain that such creatures can't have a soul, as having one is the one and only requirement to be called a living creature (animals and plants have soul, too.)
Edit: I’m not in need of creatures per se. I just can’t conceptualise how the God of Death won’t interfere if such creatures are created; her role is to help Souls to the Otherworld when they die, while the God of Birth rules the other way around.
Edit2 (the Cycle of Life): as the Parents ruled, every creature must born, from the power of Garquon, Lord of Birth. Upon a set time, which is different for all races, Ketirai, Lady of Death shall farce her to the Passage. From the Passage the Soul must find its way to Shutra (ie. the Otherworld) where it can rest, to be reborn again. When the time comes, the Soul can get back to the Planes of existence through the Passage.
Edit3: the undeads in my story ideas range from unintelligent (maybe insentient) beings used as expendable soldiers, to highly intelligent beings, like liches. I can easily conceptualise the former (animating dead bodies with magic), but the latter causes some troubles (see Edit2).
The second edit may be used to achieve what I want: some mechanics on how undead creatures can exist in a world where the cycle of life is effectively set to stone.