A new instability for a splay-bend Ising wall was found in a 5CB nematic li
quid crystal layer. This instability, which occurs in the presence of an ex
ternal horizontal magnetic field, is driven by the elastic anisotropy of th
e liquid crystal material. Depending on the homogeneity of the magnetic fie
ld, the unstable straight interface evolves towards a new steady state or u
ndergoes a spinodal decomposition into facets. Energy arguments are given i
n order to explain these physical phenomena.