DieMag633, a cast Mg-Al-Ba-Ca alloy, was the focus of this study. Brittle interdendritic phases strongly influenced the crack initiation and propagation under quasi-static and fatigue loading. Especially under tensile loading, the material showed a low resistance to failure. Selected fatigue loading sequences were applied to investigate their influence on crack propagation. DieMag633 in this study contained shrinkage cavities and pores of significant size and irregular distribution. Even though pores played a role in initiating the crack, it was mainly influenced by the Ba- and Ca-rich phases, being and staying much harder under deformation than the Mg-matrix. Apart from the fatigue crack propagation region under fatigue loading, there was no transgranular cracking found within the dendritic α-Mg grains. Only under compression did the dendritic α-Mg grains bridge the crack from one brittle phase to another. Transgranular cracking within the compact Ba-rich phase was very pronounced, starting with many microcracks within this phase and then connecting to the macrocrack. The lamellar Ca-rich phase showed also mainly transgranular cracking, but being small lamellae, intergranular cracking was additionally found. The hardness increase under deformation depended on the loading condition; a compression load strain-hardened the material the most. µCT analysis was applied to characterize the amount and location of the shrinkage cavities and pores in the individual gauge length.