  We present a detailed study of SMM 02399-0136, a hyperluminous, active galaxy
selected from a sub-mm survey of the distant Universe. This galaxy is the
brightest source in the fields of seven rich, lensing clusters - total area
0.01 deg^2 - that we have mapped with a sensitivity of ~2 mJy/bm at 850um. We
identify a compact optical counterpart with B ~ 23 and a LSB companion 3" away.
Our spectroscopy shows that components have the same redshift; z = 2.803 +/-
0.003. The emission lines widths, FWHM ~ 1000-1500 km/s, and line ratios, along
with the compact morphology and high luminosity (M_B ~ -24.0) of the galaxy
indicate that SMM 02399-0136 contains a rare dust-embedded, narrow-line or
type-2 AGN. The source is lensed by the foreground cluster, amplifying its
apparent luminosity by a factor of 2.5. Taking this into account, we estimate
that SMM 02399-0136 is five times more luminous than F10214+4724. Its far-IR
and H-alpha luminosities and LSB radio emission are indicative of an extremely
high SFR - several thousand Mo/yr. This assumes that a starburst is the
dominant source of energy, but we cannot yet determine the relative
contributions of the starburst and the buried AGN. A dust mass of 5-7 x 10^8 Mo
is indicated by our data for T(dust) ~ 40-50K, independent of whether the
dominant energy source is an AGN or a starburst. We estimate the possible space
density of such luminous sub-mm sources and find that while a very large
population of these obscured sources could be detected in future wide-field
sub-mm surveys, they are unlikely to dominate the faint counts in this
waveband. Galaxies such as SMM 02399-0136 and F10214+4724 cannot be easily
detected in conventional AGN/QSO surveys, and so estimates of the prevalence of
AGN in the early Universe may require significant revision.
