The impact of configurational entropy, enthalpy, and strain energy on the phase stability of high-entropy materials has not yet been investigated quantitatively. We used ab-initio calculations to predict the driving force for decomposition of 126 equimolar Al-containing high-entropy sublattice nitrides (HESN), which are all metastable with respect to all corresponding equimolar lower-entropy nitride phases. The entropy stabilization of ≈-0.06 eV/at at 1073 K is overruled by the 0.10-0.27 eV/at enthalpy-governed driving force for decomposition. Stabilization is however predicted for 22 compositions due to the -0.01 to -0.28 eV/at strain energy contribution caused by large differences in equilibrium volume between the HESN and their decomposition products. The predicted stabilities were validated with diffraction and tomography data of 9 annealed nitride systems. Hence, it is evident that only strain enables the stabilization of the here studied Al-containing HESN, while the entropic contribution is overruled by endothermic mixing.