The natural time scale of valence electronic motion in molecular systems is on the order of hundreds of attoseconds. Consequently, the time-resolved study of electronic dynamics requires a source of sub-femtosecond pulses. Pulses in the soft x-ray domain can access core-level electrons, enabling the study of site-specific electron dynamics through attosecond pump/probe experiments. As time-resolved pump/probe experiments are nonlinear processes, these experiments require high brightness attosecond x-ray pulses. The X-ray Laser-Enhanced Attosecond Pulses (XLEAP) collaboration is an ongoing project for the development of attosecond x-ray modes at the Linac Coherent Light Source (LCLS). Here we report development of a high power attosecond mode via cascaded amplification of the x-ray pulse. We experimentally demonstrate generation of sub-femtosecond duration soft x-ray free electron laser pulses with hundreds of microjoules of energy. In conjunction with the upcoming high repetition rate at LCLS-II, these tunable, high intensity attosecond capabilities enable new nonlinear spectroscopic techniques and advanced imaging methods. This work was supported by US Department of Energy Contracts No. DE-AC02-76SF00 and the Basic Energy Sciences Accelerator and Detector Research Program.