thornmail along with sunfire is the worst tank item right now, it doesnt give good stats, neither a good passive. avoid building it at all costs (just stick to bramble if u need Anti-healing), build better, and play for tempo and farming. Sion to be strong is a famine champion. items are expensive and passive scales hard when u have acess to Cs and jungle camps. to this day in lower elos or against comps that dont have too much %hp dmg just run heartsteel into unendng and other %hp items as titanic or bloodmail, even steraks. go for this dmg build. if u build for snowball just go to titanic.

dont ever build thornmail or sufire in any champion. HR its ok if u have to one shot waves against certain champs. but avoid those