  The QCD axion is one of the best motivated dark matter candidates. The
misalignment mechanism is well known to produce an abundance of the QCD axion
consistent with dark matter for an axion decay constant of order $10^{12}$ GeV.
For a smaller decay constant, the QCD axion, with Peccei-Quinn symmetry broken
during inflation, makes up only a fraction of dark matter unless the axion
field starts oscillating very close to the top of its potential, in a scenario
called "large-misalignment". In this scenario, QCD axion dark matter with a
small axion decay constant is partially comprised of very dense structures. We
present a simple dynamical model realising the large-misalignment mechanism.
During inflation, the axion classically rolls down its potential approaching
its minimum. After inflation, the Universe reheats to a high temperature and a
modulus (real scalar field) changes the sign of its minimum dynamically, which
changes the sign of the mass of a vector-like fermion charged under QCD. As a
result, the minimum of the axion potential during inflation becomes the maximum
of the potential after the Universe has cooled through the QCD phase transition
and the axion starts oscillating. In this model, we can produce QCD axion dark
matter with a decay constant as low as $6\times 10^9\,{\rm GeV}$ and an axion
mass up to 1 meV. We also summarise the phenomenological implications of this
mechanism for dark matter experiments and colliders.
