Gamma rays from the most extreme extragalactic emitters not only enable the probe of particle acceleration at multi-TeV energies, but they can also be exploited as tracers of the electromagnetic content and fabric of the universe. These include constraints on, or even measurements of, magnetic and photon fields populating the largest voids in the cosmos, as well as tests of fundamental physics in the form of Lorentz invariance and axion-like particles. The emphasis of this seminar will be put on exploiting the emerging population of extreme blazars as gamma-ray beacons. After an outlook of the lessons learned from current-generation observatories at GeV- and TeV-energies, this seminar will expand on the discoveries and constraints that can be expected from the Cherenkov Telescope Array (CTA). IJCLab is actively involved in the construction of CTA, in particular in the project of gamma-ray camera, NectarCAM, developed by French teams together with other European institutes. The concluding part of this seminar will be dedicated to the technical activities of the IJCLab team on NectarCAM and to the wealth of high-quality data expected from extragalactic accelerators in the first years of the CTA observatory.