FOR A LONG TIME HIGH-REDSHIFT BL LAC OBJECTS WERE DEEMED NOT TO EXIST. FERMI SHOWED US THAT THERE IS RELEVANT POPULATION OF BL LACS WITH REDSHIFT BEYOND 1.0. SOME OF THEM BELONG TO THE HSP CLASS AND ARE AMONG THE HARDEST GAMMA-RAY SOURCES DETECTED BY FERMI SHOWING EMISSION UP AND BEYOND 100 GEV. THIS MAKES THEM THE MOST LUMINOUS BL LACS EVER DETECTED AND RATES THEM AMONG THE MOST POWERFUL ACCELERATORS IN THE UNIVERSE. WE PLAN TO OBSERVE THE EXTREME BL LAC 2FHL J2146.61345 SIMULTANEOUSLY WITH NUSTAR XMM-NEWTON AND SARA. THIS WILL PROVIDE UNPRECEDENTED COVERAGE OF THE SYNCHROTRON PEAK FROM NIR TO HARD X-RAY ALLOWING US: TO UNDERSTAND THE NATURE AND THE ENERGETIC OF THIS OBJECT TO ANSWER LONG-STANDING QUESTIONS ON THE BLAZAR SEQUENCE AND TO USE IT AS PROBE OF THE EXTRAGALACTIC BACKGROUND LIGHT.