  The recently discovered material Cs$_3$Fe$_2$Br$_9$ contains Fe$_2$Br$_9$
bi-octahedra forming triangular layers with hexagonal stacking along the $c$
axis. In contrast to isostructural Cr-based compounds, the zero-field ground
state is not a nonmagnetic $S=0$ singlet-dimer state. Instead, the Fe$_2$Br$_9$
bi-octahedra host semiclassical $S=5/2$ Fe$^{3+}$ spins with a pronounced
easy-axis anisotropy along $c$ and interestingly, the intra-dimer spins are
ordered ferromagnetically. The high degree of magnetic frustration due to
(various) competing intra- and inter-dimer couplings leads to a surprisingly
rich magnetic phase diagram. Already the zero-field ground state is reached via
an intermediate phase, and the high-field magnetization and thermal expansion
data for $H\parallel c$ identify ten different ordered phases. Among them are
phases with constant magnetization of 1/3, respectively 1/2 of the saturation
value, and several transitions are strongly hysteretic with pronounced length
changes reflecting strong magnetoelastic coupling.
