The Ancient Magus Bride Vol 1

The Ancient Magus' Bride by Kore Yamazaki. A Fantasy manga rated Teen, published in English by Seven Seas.

Chise Hatori has nothing left to live for when she's sold at auction to Elias Ainsworth, a towering mage with the skull of a beast for a head. Instead of a master, she finds a teacher, as Elias takes her on as his apprentice and future bride in a world of fairies, magic and old powers.