ASOIAF: A song of an ice elf

Sigmund was always a powerful person, a dragon soul, a mage, a noble but now, his goddess asked him to be a savior once again. Only this time, he wasn't saving his world but the world of planetos. This was no evil overlord, using dark magic to enslave all mortals but a monster of ice and death, which just so happened to be his speciality. Will he kill the night king or will the long night bring death to all?

(The mc is based on a TTRPG character I played years ago)