He was charmed by Miquella and ordered him to die, so he could place his older brother, Radahn's (which is actually his half brother) soul in Mohg's body.
Go search/read series with "this" in [Summary]
Clearly there are other readers like you Nota that focus on plot/killing/etc and you should try finding this kind of series.
I didn't realize today was your birthday! You're truly a special dragon to be born on your country's Independence Day.
Happy birthday, brother.
I hope you'll enjoy the chapter coming out today!