A rational approach to designing magnetic materials requires a shift from exploratory to hypothesis-driven synthesis. Challengingly, the complex phase space of solid-state systems is such that small changes in composition can drastically change the structure of the resulting material. In comparison, pressure provides an incrementally tunable vector, serving to densify structures and shift electron density from core to valence states. Lanthanides (Ln) represent a particularly powerful class of materials for this approach as the application of pressure promotes electron delocalization, profoundly impacting observed magnetic behaviour. We aim to elucidate the orbital specific nature of pressure driven phase changes in lanthanide halides using X-ray absorption spectroscopic technique in order to deconvolute the interplay of between competing energetic states responsible for their structure, electronics and magnetism.