Recent detections of gravitational waves (GWs) from two merging black holes (BHs) by LIGO/VIRGO provide valuable information about nature of BHs and physics of gravity. In addition, recent studies also suggest that BHs may be used to probe ultralight particles such as axions as well as dark matter. In this talk, we present both single and binary black hole dynamics and their implication in gravitational wave era using numerical simulations. First BH dynamics are explored within modified or alternative theories of gravity. Multiple families of models were chosen including a family of Einstein-Maxwell-Dilaton-Axion theories and a family of models based in quadratic gravity. The stability of single BH dynamics and binary black hole (BBH) mergers are investigated in order to probe deviations from the standard GW signature of general relativity in these theories. In addition, we discuss a study of BBH mergers in extreme cases for which the mass ratio of the binaries is not close to one. Such intermediate mass ratio inspirals will provide a new sector for future GW observations as well as hints of intermediate mass BH population.