You can buy all Spells in Skyrim from the College of Winterhold, but the Master Spells must have quests completed to earn them! Once you reach  Level 90 ...

You get the Azura's Star and a pretty strong mage follower if you decide to deliver the star to Azura. You get the Black Star if you decide that you want to side with Nelacar.

These isekai anime took a turn for the dark as they feature dark themes that are not usually associated with the isekai genre.