  Ultracompact objects with light-rings (LRs) but without an event horizon
could mimic black holes (BHs) in their strong gravity phenomenology. But are
such objects dynamically viable? Stationary and axisymmetric ultracompact
objects that can form from smooth, quasi-Minkowski initial data must have at
least one stable LR, which has been argued to trigger a spacetime instability;
but its development and fate have been unknown. Using fully non-linear
numerical evolutions of ultracompact bosonic stars free of any other known
instabilities and introducing a novel adiabatic effective potential technique,
we confirm the LRs triggered instability, identifying two possible fates:
migration to non-ultracompact configurations or collapse to BHs. In concrete
examples we show that typical migration/collapse time scales are not larger
than $\sim 10^3$ light-crossing times, unless the stable LR potential well is
very shallow. Our results show that the LR instability is effective in
destroying horizonless ultracompact objects that could be plausible BH
imitators.

General Relativity and Quantum Cosmology;High Energy Astrophysical Phenomena;High Energy Physics - Theory