I have a bard that has been true polymorphing into a dragon turtle and attacking coastal towns on a large island (think "Big Island" Hawaii) I am doing this so another member of my party can be true polymorphed into a bronze dragon adult and be seen "fighting" with me.
Our plan is for the fight to go into the ocean, then sometime later he as the bronze dragon will drag a part of a dragon turtle' body out with him and declare himself their savior and new leader and/or guardian. then true polymorph on him will be dropped, he'll say he used his dragon shapechange ability to look human. Then he will say he has to keep traveling to "right many wrongs" but that one day he will return.
Then at the end of the campaign if we are still alive, I will permanently make him an adult bronze (around 200 years old) and he will get to spend upwards of 5000 years retired and living as the king of an island nation. Also so he is a sailor background fighter so he plans to slowly reshape them into a sea fairing warrior people like the ancient Maori. He also plans to take a bride from the tribe once every generation till the whole island is full of dragon ancestry sorcerers and half-dragon humans.
We may not need the corpse head of a dragon turtle to pull this deception off we could say it died below the waves but it would help give us trick them and then for centuries the skull of a dragon turtle can adore their main village as a constant reminder of why they should follow him.