  We formulate and study a spin-orbital model for a family of cubic double
perovskites with $d^1$ ions occupying a frustrated fcc sublattice. A
variational approach and a complimentary analytical analysis reveal a rich
variety of phases emerging from the interplay of Hund's and spin-orbit
couplings (SOC). The phase digram includes non-collinear ordered states, with
or without net moment, and, remarkably, a large window of a non-magnetic
disordered spin-orbit dimer phase. The present theory uncovers the physical
origin of the unusual amorphous valence bond state experimentally suggested for
Ba$_2B$MoO$_6$ ($B$=Y,Lu), and predicts possible ordered patterns in
Ba$_2B$OsO$_6$ ($B$=Na,Li) compounds.
