Undead Settlement, Dilapidated Bridge bonfire
Make your way up to the Tower where the Giant is, speak with Siegward and ride the elevator up (step on then roll off the elevator staying on the same level where another elevator will come down, ride this one up)
Once at the top, head back down and roll off onto a wooden ledge, head out and speak with Siegward. After the dialogue, head down and take out the Demon to acquire a Fire Gem