  Trapped regions bounded by horizons are the defining features of black holes.
However, formation of a singularity-free apparent horizon in finite time of a
distant observer is consistent only with special states of geometry and matter
in its vicinity. In spherical symmetry such horizons exist only in two classes
of solutions of the Einstein equations. Both violate the null energy condition
(NEC) and allow for expanding and contracting trapped regions. However, an
expanding trapped region leads to a firewall. The weighted time average of the
energy density for an observer crossing this firewall is negative and exceeds
the maximal NEC violation that quantum fields can produce. As a result, either
black holes only can evaporate or the semiclassical physics breaks down already
at the horizons. Geometry of a contracting trapped region approaches the
ingoing Vaidya metric with decreasing mass. Only one class of solutions allows
for a test particle to cross the apparent horizon, and for a thin shell to
collapse into a black hole. These results significantly constrain the regular
black hole models. Models with regular matter properties at the horizon can be
realized only if significant departures from the semiclassical physics occur
already at the horizon scale. The Hayward-Frolov model may describe only
evaporation, but not formation of a regular black hole.
