I'm looking for a fairly old manga series, it's started off with the MC caught between a battle of a princess from another world and something and got killed, she then resurrected him and bring him to her world but apparently resurrected are forbidden or maybe was very looked down upon, he attended the same school as her and work his way to recognition, along the way he obtained a sword than can transform into a girl, all I remember is that his younger brother got taken a at the end of the 1st part and the sequel started off with him now being a king

Immortal Regis and sequel Cavalier of the Abyss

~4400 manga completed
~1700 ongoing...