Hangyaku no Soul Eater

Sora Mitsurugi's life isn't going as planned. Disowned for incompetence by his renowned Sword Saint father, dumped by his fiancée, and banished from his island home, he sets off to prove himself as an adventurer on the mainland. But when his new so-called-comrades deceive him and use him as bait for a dangerous monster, he finds himself on the precipice of death. Just before his demise, a raging power courses through his body and he is reborn as the legendary dragon, the Soul Eater! Now, with his newfound strength, Sora is out for revenge!

(Source: J-Novel Club)