BL Lac Objects at the highest redshifts

For a long time highredshift BL Lac objects were deemed not to exist. Fermishowed us that there is r^ant population of BL Lacs with redshift beyond 1.0.Some of them belong to the highsynchrotron peaked (HSP) class and are among thehardest gammaray sources detected by Fermi showing emission up and beyond 100GeV. This makes them the most luminous BL Lacs ever detected and rates themamong the most powerful accelerators in the Universe. We plan to observe 3extreme BL Lacs simultaneously with XMMNewton, GROND and Swift. This willprovide unprecedented coverage of the synchrotron peak from IR to hard Xrayallowing us: to understand the nature and the energetic of these objects, toanswer longstanding questions on the blazar sequence and to use them as probes of the EBL. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]