The synthesis and characterization of the previously unknown material LaCaScIrO$_6$ is reported. LaCaScIrO$_6$ presents a new example of the rare case of a double perovskite with the strongly spin-orbit coupled 5textit{d}-ion Ir$^{4+}$ as its only magnetic species, forming a monoclinically distorted version of the frustrated fcc lattice. Magnetization measurements show a weak anomaly at 8~K. The Curie-Weiss temperature Theta$_{CW}$ and effective magnetic moment mu$_{eff}$ of LaCaScIrO$_6$ are in close proximity to the related compound La$_2$MgIrO$_6$ but differ from La$_2$ZnIrO$_6$. This suggests that the nature of the non-magnetic textit{B}-ion, namely its textit{d}-orbital filling has a strong influence on the magnetic properties. The textit{d}$^{0}$-ions Sc$^{3+}$ and Mg$^{2+}$ allow a different kind of exchange interactions within the Ir-sublattice than the textit{d}$^{10}$-ion Zn$^{2+}$. In addition, ac-susceptibility data does not show signs of a spin-glass ground state. The nature of the magnetism in LaCaScIrO$_6$ has been further elucidated using muon spin relaxation measurements. The zero-field measurements reveal the absence of well defined oscillations down to 1.6,K, while temperature dependent $mu$SR studies show an anomaly at 8,K. Overall, our results suggest the presence of two different magnetic environments or domains in LaCaScIrO$_6$, which is likely related to its structural features.
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