High-power attosecond X-ray pulses are ideal probes of ultrafast nonlinear interactions in quantum systems. We demonstrate the production of soft X-ray pulses with terawatt-scale peak powers and few hundred attosecond pulse durations in a two-stage cascaded X-ray free-electron laser. We diagnose the pulse properties in the time domain with angular streaking. Our results exceed the peak power of previous state-of-the-art attosecond XFELs by an order of magnitude. Furthermore, our data provides strong evidence of operation in the soliton-like superradiant regime of the free-electron laser at X-ray wavelengths.

X-ray free-electron lasers (XFELs) have emerged as a promising counterpart to high harmonic generation sources for scientific applications requiring high power attosecond X-ray pulses. To date, attosecond XFELs have specialized in producing isolated pulses enabling the study of nonlinear ultrafast science in the impulse regime. We present a method to coherently shape the spectrotemporal characteristics of attosecond X-ray free-electron laser pulses, offering a path towards broader coherent bandwidths and more versatile control of pulse amplitude and phase. We show that with undulator tapering in a fresh slice reamplification scheme, it is possible to produce phase-stable pulse pairs with tunability in color and temporal separation, phase-stable pulse trains, and flexibly chirped pulses. Our method enables bandwidth broadening for attosecond X-ray FELs and offers a path towards sub-100 as pulse duration at soft X-ray wavelengths.