  GRB 110918A is the brightest long GRB detected by Konus-WIND during its 19
years of continuous observations and the most luminous GRB ever observed since
the beginning of the cosmological era in 1997. We report on the final IPN
localization of this event and its detailed multiwavelength study with a number
of space-based instruments. The prompt emission is characterized by a typical
duration, a moderare $E_{peak}$ of the time-integrated spectrum, and strong
hard-to-soft evolution. The high observed energy fluence yields, at z=0.984, a
huge isotropic-equivalent energy release $E_{iso}=(2.1\pm0.1)\times10^{54}$
erg. The record-breaking energy flux observed at the peak of the short, bright,
hard initial pulse results in an unprecedented isotropic-equivalent luminosity
$L_{iso}=(4.7\pm0.2)\times10^{54}$erg s$^{-1}$. A tail of the soft gamma-ray
emission was detected with temporal and spectral behavior typical of that
predicted by the synchrotron forward-shock model. Swift/XRT and Swift/UVOT
observed the bright afterglow from 1.2 to 48 days after the burst and revealed
no evidence of a jet break. The post-break scenario for the afterglow is
preferred from our analysis, with a hard underlying electron spectrum and
ISM-like circumburst environment implied. We conclude that, among multiple
reasons investigated, the tight collimation of the jet must have been a key
ingredient to produce this unusually bright burst. The inferred jet opening
angle of 1.7-3.4 deg results in reasonable values of the collimation-corrected
radiated energy and the peak luminosity, which, however, are still at the top
of their distributions for such tightly collimated events. We estimate a
detection horizon for a similar ultraluminous GRB of $z\sim7.5$ for Konus-WIND,
and $z\sim12$ for Swift/BAT, which stresses the importance of GRBs as probes of
the early Universe.
