  We present new numerical relativity results of neutron star mergers with
chirp mass $1.188M_\odot$ and mass ratios $q=1.67$ and $q=1.8$ using
finite-temperature equations of state (EOS), approximate neutrino transport and
a subgrid model for magnetohydrodynamics-induced turbulent viscosity. The EOS
are compatible with nuclear and astrophysical constraints and include a new
microphysical model derived from ab-initio calculations based on the
Brueckner-Hartree-Fock approach. We report for the first time evidence for
accretion-induced prompt collapse in high-mass-ratio mergers, in which the
tidal disruption of the companion and its accretion onto the primary star
determine prompt black hole formation. As a result of the tidal disruption, an
accretion disc of neutron-rich and cold matter forms with baryon masses
${\sim}0.15M_\odot$, and it is significantly heavier than the remnant discs in
equal-masses prompt collapse mergers. Massive dynamical ejecta of order
${\sim}0.01M_\odot$ also originate from the tidal disruption. They are neutron
rich and expand from the orbital plane with a crescent-like geometry.
Consequently, bright, red and temporally extended kilonova emission is
predicted from these mergers. Our results show that prompt black hole mergers
can power bright electromagnetic counterparts for high-mass-ratio binaries, and
that the binary mass ratio can be in principle constrained from multimessenger
observations.
