My intent is specific to the following: Cursing someone to become an animal, even if just big adjustments to appearance and internal nature. Then with a secondary effect of influencing next reincarnation by having that person continuously reincarnate into that kind of beasts womb.
Personal reasoning behind this is irrelevant other than it would be extremely fitting for the target and would be their worst fears combined with loss of agency and control. Combined with the fact the target detests fat people and fears weight gain themselves.
One example might be a curse to transform the target into a pig or enhance piggishness, or likewise a large blubbery cumbersome creature like a hippo.
Any ideas on where and how to start? Whether through spirit possession or invoking a demon?