Lorewise it is impossible to reverse being undead but still and adoption sure it's one way for zombie to get's living heir I suggest another thing: chance to adopt normal living heir. Need zombie/demon romance because some zombie/demon have very warm heart.

starting as demonic character but tired being demon want to play as succubus but because no seduction no one being cuckcolded feels kinda bland because demon supposedly full of debauchery and sins change religion and start fighting for the good guys adopted child still a demon.

prolong fight in fel territory having fel corruption raised to 100 and become demon this can be avoided by resign as commander.

play on second start date and gets undead plague while you have no living being heir, adoption always result in zombie character heir.

your only child get captured by lich king and he decide to make him/her zombie, adoption always result in zombie character heir.

Suggest for author to make rare event to deal with this issue to remove demonic/undead trait. Need way to reverse undead/demon status since there is none, due to it's mechanic undead and demonic character cannot produce heir using natural means due massive fertility malus even though they changing religion to non death god or non fel religion and their adopted children always be undead/demon. Describe your suggestion in full detail below:

Child of destiny ck2