Despite its name, Dragon Slayer Magic is not magic created to slay dragons; it is the magic of dragons. Choose one element (fire, water, ice, lightning, etc.) to become your element, and you gain immense resistance to it to the point where only a superior wielder of your element can harm you. For instance, a fire dragon Slayers can swim in lava with no issue. You can consume your element to regain your reserves and give yourself a temporary boost in power, but this power is not without costs. You will slowly but surely transform into a dragon and slowly but surely lose contact with your humanity. Even if you possess the ability to transform into a humanoid form, you will not be able to enjoy the pleasures you enjoyed as a human.
Project V17/Powers/Shardless/Magic and Mystic Powers/Dragon Slayer Magic (Fairy Tail)
From Worm CYOA Wiki
Dragon God Slayer Magic (Fairy Tail)
You possess the ability to combine the powers of a dragon and a god. This halts your transformation into a dragon and allows you to wield both Dragon Slayer Magic and God Slayer Magic together, allowing you to combine the two for greater effect and more potent attacks.