Straid is located in Lost Bastille after the Ruin Sentinels near the bridge to Sinner's Rise, in the opened jail cell. Initially he is petrified, surrounded by five rupturing hollows that deal physical damage. A Fragrant Branch of Yore is required to cure him. One can be obtained at the bottom of The Gutter, immediately before the fog gate to Black Gulch. Once you cure him, he will sell the spells and offer you trading Boss soul services.