Fantasy/ Dystopian book where hero is a King and heroine has to kill him. Spoilers ahead.

[Sammi]

Dystopian/fantasy where heroine's family is the leaders of a resistance, she is sent in to kill the "unkillable king." Really it's that he takes a pill to stay immortal and has a machine that speeds up his healing (or maybe it's a cryogenic thing). They fall in love. It's a series of books and at one point she's also frozen like 100 years or something.

Read this book in like 2022-ish.