Soul Gem Collector3 books in series
Soul Gem Collector Publisher's Summary
From book one: Being able to rip the soul out of a creature, store it in a precious gem, and use it to cast spells might make me the most powerful student at the Astra Argent Magical Academy. Only I can’t tell anybody about it.
Because it's illegal, and they’ll kill me if they find out.
But they need me whether they know it yet or not, so I’m going to capture a lot of souls and cast a lot of spells.
This is gonna be a crazy semester.
This is a monster-girl harem dark-magic academy story.