Additively-manufactured metastable high entropy alloys are emerging as promising alternatives for energy absorption in protective equipment due to their exceptional ductility. In this study, an elongation of over 50 % was achieved in a directed energy deposition fabricated Fe45Mn35Co10Cr10 metastable high entropy alloys (MHEAs). The inherent high cooling rate and repeated heat treatment in the directed energy deposition process were harnessed to deliberately induce an uneven distribution of Mn elements, forming a cellular structure with chemical boundary. The unique heterogeneity stacking fault energy results in a synergistic interplay of multiple deformation mechanisms such as, dislocation slip, transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP). In the initial stage of deformation, dislocation slip and TRIP contribute to the main plasticity. Stacking faults/martensite are confined within the cellular structure with low stacking fault energy (SFE). As strain increases, TWIP and TRIP are activated in the high SFE regions. Twins and martensite across the cell wall were observed. As strains exceed 40 %, primary martensite variant and twinning with a particular intersection angle take place to further enhance the deformation ability. The chemical boundary with stacking fault energy heterogeneity not only simultaneously activates both TRIP and TWIP, but also maintains the martensite and twins at the nanoscale.