One prediction of ΛCDM is the existence of partially phase-mixed substructures from accreted dwarf galaxies in the Milky Way stellar halo. Substructure originating in a single accretion event can be readily identified as a tight cluster of stars in phase space with similar chemical properties. Recently, the discovery of the Gaia Sausage Enceladus (GSE) has revolutionised our understanding of the complex assembly of the Milky Way halo. We present a review of the chemistry that characterises the last major merger that happened to the Milky Way some 9-10 Gy ago.