Reona the Changing Hero (変身勇者リオナ)

Reona’s party made it to the fortress of Demon Lord Stenga before getting wrecked by tentacles and reduced to Level 1.
With her last bit of strength Reona delivered a critical hit on Stenga with a poison-tipped sword. Her team swept in and, spending a portion of their own life, revived Reona and fled from the demon castle to the sea, and then to a solitary island where men gather.