The Partnership for Advanced Computing in Europe (PRACE) awarded 25 Million core-hours to a EPP researchers team lead by T. Grismayer to explore in the laboratory pair-dominated plasmas under the presence of ultra-intense fields and mimicking extreme astrophysical scenarios resorting to the unprecedented power of future laser facilities. This proposal aims to exploit the unique computing facilities provided by PRACE to address these exciting challenges by leveraging on the recently pioneered advances on ab initio simulations of quantum electrodynamics (QED) effects in extreme plasmas with particle-in-cell (PIC) simulations. Another key goal of this proposal is to investigate the radiation signatures in the Compton Regime, a regime that is not accessible with standard electromagnetic methods.