The Deep Extragalactic VIsible Legacy Survey (DEVILS) is a deep, high-completeness multiwavelength survey based around spectroscopic observations using the Anglo-Australian Telescope's AAOmega spectrograph. The survey covers $\sim 4.5$ deg$^{2}$ over three extragalactic fields to Y$_{\mathrm{ AB}}< 21.2$ mag and probes sources at $0< z< 1.2$, with a median redshift of $z=0.53$. Here, we describe the DEVILS spectroscopic observations, data reduction, and redshift analysis. We then describe and release to the community all DEVILS data in the 10h [D10, COSMOS (Cosmological Evolution Survey)] region including: (i) catalogues of redshifts, photometry, spectral energy distribution fitting for physical properties, visual morphologies, structural decompositions, and group environments/halo masses, (ii) matched imaging in 28 bands from X-rays to radio continuum, and (iii) reduced 1D spectra. All data are made publicly available through Data Central (datacentral.org.au). Within D10, we obtain 5442 new high-quality spectroscopic redshifts. When combined with existing, lower quality, redshift information ($\mathrm{ {\mathrm i.e.}}$ photometric redshifts), this is increased to 7946. Of these, 3122 have a spectroscopic redshift from another source (many that was not available at the time of the DEVILS observations). As such, DEVILS provides new unique high-quality spectroscopic redshifts for 4824 faint sources in COSMOS. This increases the spectroscopic completeness at Y-mag $\sim$ 21 from $\sim$50 per cent in other samples to $\sim$90 per cent in DEVILS. Finally, we show the power of this data set by exploring the suppression of star formation in overdense environments, split by morphology and stellar mass, and highlighting the ubiquitous nature of environmental quenching.