After you complete the Flame Temple and speak to the sage Crimson in the
Raejack Inn, go to the Raejack blacksmith shop, and talk to the man on
the right. Then, go to Fina's Inn, and speak to the man by the entrance
to receive the Rust Sword. Bring this back to the man on the right at
the Raejack blacksmith shop to get the Gladius.