Severe plastic deformation response of a face centered cubic (FCC) twinning induced plasticity (TWIP) high entropy alloy (HEA), Fe 40 Mn 40 Co 10 Cr 10 , subjected to high-pressure torsion (HPT) is investigated. The so-called TWIP HEA demonstrated an extensive transformation induced plasticity (TRIP) effect even in ½ turn (shear strain, γ = 15) of HPT processing, which increased further with increasing the number of turns to 2 ( γ = 68). Additionally, HPT induced nano-structuring and heavily dislocated structure; dislocation density was of the order of 10 15 m −2 . c/a ratio of the transformed HCP phase was found to be <1.633 and it did not change with increasing the extent of shear strain. This was manifested as the occurrence of at least 50% non-basal slip in the HCP phase. For the first time, the fraction of <c> and <c+a> dislocations are quantified and their evolution are discussed in the purview of the studied alloy. The micro-mechanism of strain accommodation is correlated with increasing hardness of the HEA upon sequential HPT processing. The present work provides a viewpoint that the deformation induced HCP phase in a metastable FCC HEA can have tailored c/a ratio which triggers non-basal slip, leading to a strong and ductile material. • Microstructure evolution of a so-called TWIP HEA subjected to HPT processing was investigated. • Extensive TRIP effect and nano-structuring were observed. • A c/a ratio< 1.633 of the transformed HCP phase enabled co-occurrence of basal and non-basal slips. • The mechanisms of extensive strengthening during HPT processing were identified.