  We present a new exploration of the cosmic star-formation history and dust
obscuration in massive galaxies at redshifts $0.5< z<6$. We utilize the deepest
450 and 850$\mu$m imaging from SCUBA-2 CLS, covering 230arcmin$^2$ in the
AEGIS, COSMOS and UDS fields, together with 100-250$\mu$m imaging from
Herschel. We demonstrate the capability of the T-PHOT deconfusion code to reach
below the confusion limit, using multi-wavelength prior catalogues from
CANDELS/3D-HST. By combining IR and UV data, we measure the relationship
between total star-formation rate (SFR) and stellar mass up to $z\sim5$,
indicating that UV-derived dust corrections underestimate the SFR in massive
galaxies. We investigate the relationship between obscuration and the UV slope
(the IRX-$\beta$ relation) in our sample, which is similar to that of
low-redshift starburst galaxies, although it deviates at high stellar masses.
Our data provide new measurements of the total SFR density (SFRD) in
$M_\ast>10^{10}M_\odot$ galaxies at $0.5<z<6$. This is dominated by obscured
star formation by a factor of $>10$. One third of this is accounted for by
450$\mu$m-detected sources, while one fifth is attributed to UV-luminous
sources (brighter than $L^\ast_{UV}$), although even these are largely
obscured. By extrapolating our results to include all stellar masses, we
estimate a total SFRD that is in good agreement with previous results from IR
and UV data at $z\lesssim3$, and from UV-only data at $z\sim5$. The cosmic
star-formation history undergoes a transition at $z\sim3-4$, as predominantly
unobscured growth in the early Universe is overtaken by obscured star
formation, driven by the build-up of the most massive galaxies during the peak
of cosmic assembly.
