High-brightness sources of coherent and few-cycle-duration light waveforms with spectral coverage from the ultraviolet to the terahertz would offer unprecedented versatility and opportunities for a wide range of applications from bio-chemical sensing to time-resolved and nonlinear spectroscopy, and to attosecond light-wave electronics^,. Combinations of various sources with frequency conversion^, and supercontinuum generation^{, , –} can provide relatively large spectral coverage, but many applications require a much broader spectral range and low-jitter synchronization for time-domain measurements. Here, we present a carrier-envelope-phase (CEP)-stable light source, seeded by a mid-infrared frequency comb^,, with simultaneous spectral coverage across seven optical octaves, from the ultraviolet (340 nm) into the terahertz (40,000 nm). Combining soliton self-compression and dispersive wave generation in an anti-resonant-reflection photonic-crystal fibre with intra-pulse difference frequency generation in BaGa₂GeSe₆, the spectral brightness is two to five orders of magnitude above that of synchrotron sources. This will enable high-dynamic-range spectroscopies and provide numerous opportunities in attosecond physics and material sciences^,.