  This paper studies the whole process of gravitational collapse and accretion
of a massless scalar field in asymptotically flat spacetime. Two kinds of
initial configurations are considered. One is the initial data without black
hole, the other contains a black hole. Under suitable initial conditions, we
find that multi-horizon will appear, which means that the initial black hole
formed by gravitational collapse or existing at the beginning will instantly
expand and suddenly grow, rather than grows gradually in the accretion process.
A new type of critical behavior is found around the instant expansion. The
numerical computation shows that the critical exponents are universal.
