Grindelwald need Theseus's support to take over the magical world so he offers him his most dark and desperate desire, Newt. Newt is magically age reversed to a teenager back when he idolized Theseus and willingly listened to him. It's all to easy to make him listen and to have Newt let Theseus kiss him, even spread him out on the bed.
Grindelwald constantly visiting and encouraging incest and depraved acts, encouraging Theseus to use dark magic to bind Newt to him and for Newt to be a good child to his brother.
Theseus/Newt, incest, spell-non con, age regression,
Newt is magically age reversed to a teenager back when he idolized Theseus and willingly listened to him. It's all to easy to make him listen and to have Newt let Theseus kiss him, even spread him out on the bed.
Grindelwald constantly visiting and encouraging incest and depraved acts, encouraging Theseus to use dark magic to bind Newt to him and for Newt to be a good child to his brother.