High temperature deformation behavior of a heat-resistant duplex stainless steel, used as a retort in the Pidgeon process for Mg production, was investigated in this study. 25Cr-8Ni based duplex stainless steels were cast into rectangular ingots, with dimensions of $350mm{ imes}350mm{ imes}100mm$. Nitrogen and yttrium were added at 0.3wt.% each to enhance the heat-resistance of the steel. Phase equilibrium was calculated using the thermodynamic software FactSage$^{(R)}$ and the database of FSStel. For comparison, cast 310S steel, a widely used heat-resistant austenitic stainless steel, was also examined in this study. Dilatometry was conducted on the as-cast ingots for the temperature range from RT to $1200^{circ}C$ and the thermal expansion coefficients were evaluated. The nitrogen addition was found to have an effect on the thermal expansion behavior for temperatures between 800 and $1000^{circ}C$. High temperature tensile and compression tests were conducted on the ingots for temperatures ranging from 900 to $1230^{circ}C$, which is the operation temperature employed in Mg production by the Silico-thermic reduction process. The steel containing both N and Y showed much higher strength as compared to 310S.