The Curse of Yondalla is an especially powerful cleric spell, granted only to followers of the halfling deity Yondalla and reserved only for use against creatures guilty of destroying a halfling community or slaying innocent and defenseless halflings.
When cast, anything affected by the spell is stunned so that it cannot make any offensive actions and then over the next minute, the affected person or creature will revert to infancy, first slowly and then faster. Undead creatures without any sort of divinity are immediately destroyed while extraplanar creatures are banished to their home plane and unable to return for up to 100 years.
The child is then brought up by a halfling community or the priest who cast the spell and taught to respect halflings. If the child is an inherently dangerous species, it is sent to foster parents who would be able to handle such a creature and raise it to be moral and ethical.
The only way to reverse the effects of this spell is for the cleric who cast it or a good-aligned cleric of a higher level than the caster to cast a remove curse spell upon the infant otherwise, only a wish can dispel the effects. Destroyed undead creatures can also be brought back but a banished outsider requires a wish or some kind of divine intervention to be able to return.