Among the most important and debated problems in astrophysics and cosmology is the formation of supermassive black holes and their feedback onto the surrounding environment. How fast can black holes accrete matter? How do they influence their host galaxies? High-energy astrophysics and, in particular high-resolution X-ray spectroscopy, provide an excellent workbench to address these problems. Here I will review recent efforts and progress in the field through the study of extreme phenomena such as super-Eddington accretion and outflows.