First, a large fire must be built with the dragons' skull in the center, which the target will sit on top of.
The target then consumes the feather while making a saving throw against the caster's Spell Save DC.
The type of saving throw is the same as the caster's spellcasting ability.   If successful, the target becomes a Demi-god, gaining the Partial Divinity feat.
On a fail, the target has their maximum HP permanently reduced by 75%. This effect bypasses all magic effects and can only be reversed by a wish spell.   Alternatively, you can brew a Potion of Divinity by targeting an Estus Flask with the spell and sticking the feather into the flask.
The caster must succeed on a saving throw against their own Spell Save DC or have their maximum HP permanently reduced by 75% and fail to create the potion.
This effect bypasses all magic effects and can only be reversed by a wish spell. The type of saving throw is the same as the caster's spellcasting ability.   Ascended Spell. This spell costs 2 Spell slots to cast. An 8th-level spell slot and a 7th-level spell slot.