  We establish the double perovskite Ba$_2$CeIrO$_6$ as a nearly ideal model
system for j=1/2 moments, with resonant inelastic x-ray scattering indicating a
deviation of less than 1% from the ideally cubic j=1/2 state. The local j=1/2
moments form an fcc lattice and are found to order antiferromagnetically at
$T_N$=14K, more than an order of magnitude below the Curie-Weiss temperature.
Model calculations show that the geometric frustration of the fcc Heisenberg
antiferromagnet is further enhanced by a next-nearest neighbor exchange,
indicated by ab initio theory. Magnetic order is driven by a bond-directional
Kitaev exchange and by local distortions via a strong magneto-elastic effect -
both effects are typically not expected for j=1/2 compounds making Ba2CeIrO6 a
riveting example for the rich physics of spin-orbit entangled Mott insulators.
